You can't do this in unity but you can put your packages into a unity package extractor: https://peraperavrc.github.io/package-extractor/
and download the fbx's from there, put them into Blender to have all the fbxs on one armature and then export that as an fbx to put back into unity. But I'm not sure why you would really want to as Booth assets are extremely user friendly and mostly made to be dragged and dropped.