Hi Elizabeth- I'm not an attorney and not on the zoning board anywhere- but a few practical thoughts-
1. Is it a private septic system? If so, you likely couldn't add any bedrooms anyways. If its town sewer, that alleviates that worry.
2. Can you pull a permit to merely "expand" one of the existing apt. units? Adding a bedroom, living room, and a full bath in the basement isn't necessarily illegal. Adding a kitchen/ kitchenette- well, maybe.
All that said, the other answers below I would agree with in general. Towns do not officially want to set a precendent by allowing you to bend the laws a little bit. You can bet that others will find out and be asking why they cannot do it as well- and that is the worst case scenario for the Town's workers and boards.
Thanks, and good luck,