I am currently subleasing an apartment legally. My girlfriend is going to move in with me but we want to make it official as she needs a proper address for her work permit. However, the problem is that the original contract with the agency as well as my contract with the main tenant is only for one person, even though the apartment is big enough for 2 people.

Even though main tenant is helpful and has no problems with it, I want to know if the agency can refuse to let me add her name officially. What are my my rights here and what is the right procedure to do it.

cheers !