How to re-use a skeleton and produce more animations and be compatible with existing animations.

Hey guys, so I want to save me some time and effort by getting an animation pack off of the internet and “re-use” the exported armature from the pack (in Blender or Maya) to produce more animations that are compatible with the skeleton that comes with the pack.

But when I import the new ones in UE4, it says that it’s not from the same armature/skeleton so i cannot use them together e.g. in a montage or blueprint.

So is there way to re-rig (or something) with e.g. the exported UE4’s skeleton (that has a lot of animations I can use) and add IK/FK/root bone e.t.c. in Blender (or Maya) so when re-imported it in UE4, my animation’s skeleton will match with the other one and work together? Or I can’t avoid from making it all from scratch ?