Root motion is calculated from the motion of the very highest bone in the skeletal hierarchy (the literal root, of which all other bones are children). The engine measures the delta of the location of that bone, removes it from the motion of the bone itself, and applies it to the actor which has the skeletal mesh instead.
Every skeleton, by definition, has a single root. So it SHOULD be retargetable, at least in theory, because there is no such thing as a skeleton without a root bone (and therefore incapable of root motion). The issue becomes whether or not UE4’s retargeting is intelligent enough to retarget from one root to the other, especially if the roots are in different places (usually root bones are at 0,0,0 but sometimes they’re placed at the hip instead) and can’t speak to that as I’ve never tried.