I think kit editing will be mostly in game - the attached is the Australian kit - which would then be masked with the particular colours needed for the resulting kit via either an in game editor or team config files.
So basically, you could probably edit the style of the lines and locations, but kits like the Indian one that have that gradient effect that probably needs a direct texture might be more difficult - unless I've just not found that bit yet.
Likewise I think sponsors would be layered on by the game as part of the team process.
Basically, like faces, you need to think about kits from the editor's standpoint, not how it's been done in the past.