[HELP] Adding entities / brushes to mods?

So for example if I where to make a teletuby mod, is there a way to add custom player entities or even just make a skin for player characters? So in a nutshell is there a way to make people ingame look different e.g teletubys?

Thanks in advance

You cant add custom skins, but you can make people cars/trees & stuff thats already in the game

EWWWWW Teletuby whats wrong with u man 3 years old?

Like in Left4Dead with the Teletubbies mod. Loads of fun blasting their faces off. /yea