How to create .fbx animations in Unreal Engine ?

I want to create some custom animations, starting from a fbx character rigged using Mixamo. I heard that I can use Maya for this, but my computer is not able to run Maya or 3DS, so, I want to ask you if there is any free plugin for UE4, that can create fbx Maya-like animations (using keys to indicate the position of each bone at a specific time)
Thank you!