UVs cannot be edited via UE4. There is no plugin that has been created yet by anyone. UE4 can generate lightmaps though for your second UV channel. This will only repack the UV used for the texture though. So, it will not break any seams or flatten any things out that is using a projection UV method, which can lead to overlapping UV errors. It’s best to do the UVs in or whatever program that may be used to handle UV Editing.
The objects that need a lightmap really need need to be unwrapped. Flatmapping can work OK for smaller objects or ones that you may not need to worry about as much, but for your “Hero” objects that need the attention really need to be unwrapped in ways that get the best results.
This will walk you through the process:
https://wiki.unrealengine/LightingTroubleshootingGuide#How_do_I_generate_lightmap_UVs_in_the_editor.3F
Essentially this is a RE-PACKING of your texture UV. It will take the UV shells that have been placed and repack them to have proper padding/spacing between other UV shells based on the minimum LM resolution you select. To reiterate again form earlier, if you have a lot of UV shells this will mean that the smaller the UV shell has to be in this space will be the less amount of resolution applied to that shell. A resolution of 32 or 64 would be really low for a large object broken up in pieces. Each one of those pieces would only be getting a minuscule portion of the LM texture space.