I'd like to see that too. It's great that we can convert Voices for Montage use - as I've done with most of my custom S90XS sounds, but being able to import custom Performances would be cool as well. Not sure how that would be accomplished though, or if it is indeed workable. In the meantime, I've manually recreated a few of my favorite S90XS Performances, and some new ones as well. Montage Performance Merge is a super-helpful feature.

As far as I can see, it's only a matter of reorganizing data... If Performance Converter remembers all Performance parameters (i.e: FX, inserts, splits, layers etc.) and Motif XF User Voices - if used instead of Presets - are copied into Montage User Area, the only operation needed is reassigning these Voices to the Performance.
I'm not sure it's like drinking a glass of water, but I feel this operation is not impossible to accomplish.
Anyay, reconstructing froms scratch Motif XF Performances in Montage is time consuming, but with the help of a good converter software it might be possibile.