Model question

[Mudkip44]

what im looking to do is change/add/replace things on a model (mainly for l4d2) im capable of getting the model into the right file formats and saving them the right way isnt the issue. ive tried changing things about the models like hair length and other things but i want to know if theres a tutorial somewhere that shows you how to add other models or change things within the current one. when ive done it before it didnt change anything in the game, any help??