I have a bunch of packages from Unity, and they have a ton of models I’d love to use for UE4 via Maya, 3DS etc…
Any ideas?
As long as they are, or can be converted to, fbx or obj formats i dont see why not.
Any idea on what the conversion process would look like? Do I need to get myself unity?
As far as I know you need unity -> search on google for possible ways e.g ://answers.unity3d/questions/449364/is-there-a-way-to-export-your-game-scene-as-an-fbx.html 
I got some free Unity Assets, and imported them directly - jut took the .fbx and imported. One difference is the proportion - you have to multiply it by 100 to work nicely, but it works. However others I couldn’t. Examples:
Worked:
https://www.assetstore.unity3d/en/#!/content/744
https://www.assetstore.unity3d/en/#!/content/63
Didn’t work (directly):
https://www.assetstore.unity3d/en/#!/content/53
The last one didn’t work probably because I was too lazy to try and make it work, since I was just trying.
I have the PropsAnimations (all in .fbx) which I’ll try to make work in UE4. I’ll keep you all informed 
PS.: I of course needed Unity to download the assets. But once they were in my hard-drive I just imported them from UE4.
the main problem with this animations is that maded for mecanim rigs.
A utility that I’m using is this:
https://www.assetstore.unity3d/en/#!/content/4031
can export in. dae, then you can import them into blender, apply scaling as desired, and export it as a FBX.
Nice. Will try it. Thanks!
I use both engines a lot and like to use most of my assets both ways… I’m a bi engine user 
You can easily use the usable formats that are within your .unitypackages
You cannot use .unitypackage files themselves*as a file; it is a proprietary Unity format unusable outside Unity.
Ex: Import your .unitypackage into Unity> then go in the asset folder(from folder path outside of unity) and copy usable formats you desire.
You can use: 3d models(.fbx .obj even .dae if you import in 3d package and re-export as fbx/obj), any images and ‘some’ animations(only those that are .fbx or .dae*requires fbx conversion) **And maybe later scripts too depending on Ue4 scripting support in future
*WARNING all models must be re-scaled for Ue4 * this can be a problem with animations caused by re-scaling rigs. I always suggest importing into your preferred 3d software and adapting assets there for Ue4 export.
You cannot use: .anim … No particles either * so if you want to remake your particle effects you have to grab/copy all the images used and reproduce it manually in cascade.
It doesn’t matter if it’s made for mecanim in the slightest other than not being able to export animators or states from the state-machine, mecanim matches your rig to its inbuilt rig for its animation state machine/blending features. You can’t export your mecanim states if that’s what your talking about.
As long as it’s .fbx files it will work in Ue4.
Other ‘tricks’ :
So, would you like to use your Unity trees in Ue4? Well you can!
You can export .obj’s from Unity with this ://wiki.unity3d/index.php?title=ObjExporter
Just drag the object/s(trees also) you want to export into the scene view and use the export option from the menu. The commenting in the code tells you where it gets exported. And again; I suggest bringing the .obj in your preferred 3d software for Ue4 conversion work(scaling, smoothing, LODs, animations, etc, etc)
&stc=1
I’m not sure if you can use assets created by other people for Unity or other 3D game engines in UE4.
It depends. If you’re talking about free Unity** specific **assets (like Unity-chan*mascott, kyle robot, angry bots, the unity spy game, all unity free assets provided & other Unity default assets) I really suggest NOT using any of them since they are widely known and ridiculous to use outside Unity tbh. For community free assets I suggest asking the provider users of said assets out of respect.
But as for assets you bought, especially 3d and animations; they are yours. Unity Technologies has no claim of ownership on those or what you do with them[edit]except don’t resell assets out of a game obviously. That aside make sure there are no legal issues with “licenses” of the assets(example: most frameworks have single-user license) you bought which in the case of 3d models and animations isn’t an issue unless the seller specifically says somewhere or by license “for Unity only”.[edit] You should also contact the seller of the asset to ask.
[EDIT 10:18pm] Just needed to add this on the side for any new devs after thinking about this during supper
Personally I don’t recommend using any un-generic*noticeable free assets ever (not in background or not just a generic prop). If you(anyone) for some reason can’t/won’t take time to do the work or pay for it by using free widely-over-used assets to sell a game then you should take some time off to learn how to model or to get money aside to pay for someone to do it for you.
Do you(anyone) want to end up as the next Air-Control? Hard work & patience is worth it. especially in this field of work. Rushing leads to falling and copy-paste games. Take your time and make what you love, and even if it doesn’t sell much at least you’ll be proud and there will always be those that appreciate it.
lol sorry when I say ‘you’ I’m not talking to anyone specific… bad habit
Unity does not ask developers / artists to sell their assets in the Unity asset store exclusively.
://forum.unity3d/threads/selling-on-the-unity-asset-store-earn-70-each-sale-non-.78039/
In other words, your Might be buying the same asset that is sold in Unity, anywhere and in any form that the seller has chosen or accepted.
If you have doubts is always good to send an email to the developer / artist who would like to buy an asset to ask any questions you may have.
At the moment they always answered me in the same way, its okay use purchased assets wherever and however you want.
Alex.-
I think whawt you could do is open the unitypackage in unity and they just browse to the project location and take the models and everything out of there then.