Hi Nicola,
I absolutely love
Disney's Old Key West Resort. The atmosphere is so relaxed. It is a wonderful place to spend time on your vacation.
I have done what you are considering doing, albeit at different resorts. I have added my mother as well as my in-laws to part of a trip at Disney's BoardWalk Villas, and my brother for part of a trip at the Cabins at Disney's Fort Wilderness Resort. As long as the total number of people does not exceed the maximum number allowed in your accommodation, you should be fine. A two-bedroom villa at Disney's Old Key West Resort sleeps up to nine guests, so you should be well within the limit.
If you booked this reservation as a Disney Vacation Club member, call Disney Vacation Club Member Services at 1-800-800-9800, and the cast member there can help you add the additional guests to your villa. If you booked directly through Disney, call them at 407-W-DISNEY (407-934-7639), and the cast member there can help you with this addition.
I hope that this information is helpful for you, Nicola! Please do not hesitate to ask if you have any further questions.