Other than guess work with modifier stacks, currently there is no clear cut way to just take these and sculpt the correctives and have them work with Unreal, simply because Unreal does not support additive morphs this way, Max’s skinmorph plus all the FFD’s angle deformers of the skin modifier are useless with Unreal, it simple would not recognize them.
This is something we are looking into for our pipeline as well. What we need is a script or plugin that would do the trick, something similar to Maya’s “extract deltas” ( /D87dbE8oLbA ), there are a few scripts floating around for max but they all failed to yield results, the only one I heard of that used to work was called MorphX2 discontinued after 2011. was free but can’t find it anywhere so far.
Looks like we have to keep looking for options.