Wat, PKHeX allows you to edit party data. It doesn't allow Battle Box editing, for obvious reasons.Unfortunately this is not possible (in fact even PKHex has the party section editing disabled)
The reason is that modifying the party data in a certain way makes the game crash. The reason is in the way the game stores the party data (i won't go into the technical details since they are really boring)
Party data is just current stats at the end, encrypted with the same seed. Hax even allows you to change the stats to whatever you want instead of calculating.