In the latest version of Addictive Football you can edit the team names in the game but that's it as far as customization goes. So I'm currently working on an update so you can edit the player names for each team.
When you now play the game the players' name will appear above his head. There will be default ones supplied with the game but you can then go into the lovely editor I've just written and change every single name for every team.
Here's how the editor looks so far:
It's really easy to use. You just click on the Team you want to change (left side of screenshot) and then you can just type over the original name. Clicking the Save button (bottom right) obviously saves your changes.
If you don't like having the players' name above the heads in the game then there's always the option to turn the feature off. Hopefully we should have the new version with the editor ready for a release tomorrow :)