An option file does all that. People create option files that fix kits and names and stuff like that. Some people also create extra teams, and players. You'd have to ask some of the people who are creating teams(I may get in trouble for that). You'll be very lucky lookin for the teams you want. You won't be able to do the correct the kits easily yourself. You need special software. I have no idea what software they use or how they do it. They live in the shadows, ha. There are subforums for option files on this forum. Check them out.
You could create the teams and players yourself, with the in game options I think. But that is a giant pain in the ass and the kits won't be correct. You'll also have to build the players faces yourself and everything else about them.