How i am working on one atm.
Start with backing up a save file etc you want to work with.
Open Teams.dbf of the mod you want then the same file of your backed up save game. Add the teams change ids TeamNum if needed.
Open players.dbf of the mod open your backup save players.dbf add the ones you need make sure if you changed any TeamNum in your Teams file change it in the players.dbf to match under team.
Once u get that far now you will need to change playerid for all the players you added, pretty simple just count up and change the ids so they dont conflict with the ones the game has used already.
Now the really fun part, now you need to add hotspot #s for all the players you added, if no hotspot info at all the game will crash, so the quick way to do it will be to look for players with similiar styles to the one you need get their hotspot numbers use them for your added player.
The accurate way would to be research and wait until someone translates what all the hotspot numbers correspond to, which if it's been done i haven't seen it yet.
There might be some other files to compare and contrast to the mod like locations, teamgear, coaches etc, but think the above is the most involved part of the process.
I have yet to do the hotspot part for the mod i am converting for my game but the above is what i have worked out sofar hope it helps, if anyone got any other info feel free to add

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