1+,
Steams source engine is only allowed to be modded unless your a retard that presumes he/she has enough money to buy a license for the engine.
It seriously is easy once you learn the curves of learning that ARE essential for a full modification(Modeling, Skinning, UN-wrapping, Compiling, Mapping, Coding).
Plus you'll get more "Bonus points" if you make a mod with source, with a team, and putted it on your CV, instead of saying "I made a game with FPSCreator"... 'Cause that'll make you look like you did nothing out of the normal.

"Your greatest teacher is your harshest critic"-'Butterfingers'