Look at the Sunny Smiles companion, Ulysses, or some of the others that make existing NPCs become companions. Actually, the Joanna companion might be a good one to look at for this too.
It'd be pretty easy to create a new variable "IsHired" that if 1, excludes all usual dialogue, and unlocks the companion stuff. You could also use that same variable for your trust system, or create a whole new one that further unlocks additional dialogue. Just have all of those additional dialogue pieces have >= numbers and have those actions add to the variable. A ton of companions use point systems like this, and once you figure it out, it's the easiest thing in the world to build off of.