(i assume your talking about ones that arent already in the game)
go into the saves folder of what ever save you want to have this applied to (if you want the jersey for all ur saves/rosters your going have to edit em all, but i suggest just editing the roster you use most)
open teamsgear.dbf
Copy a row in the teamsgear.dbf which uses the gear color you want this jersey to use to the bottom
Change the ID number to one that isnt used
Change the Team to the team you have the jersey for(
Team IDs)
Change the jerseytype to:
0: Home Jersey
1: Away Jersey
2: Alternate Jersey
3: Classic Jersey (Home)
4: Classic Jersey (Away)
5: Classic Jersey (Alternate)
6: Practice Jersey (Home)
7: Practice Jersey (Away)
If you got a file like pr03 then change the SHPNAME to pr03 and your done...do the same if its a jersey like cl06 and so on