If you downloaded a "V5 Fixed" version, the slots may already be pre-configured. If you are adding characters manually:

This is the core of your search. Let’s fix the issues so you can play as the new roster.

Usually shared through "pinned comments" or descriptions in showcase videos by creators such as DNUG or LAXA MUGEN . New Characters and Roster Expansion JUMP FORCE - All 40 Characters FULL ROSTER (Full Game)