Pictures really help. If they are manufactured Trusses, not joists, the short answer is no. If they have metal plates holding the members together, they would be trusses. As for expanding your store, you could if you own, and only you are willing to properly pull permits and have a licensed reputable contractor do the work. That would also mean that your store would have to be closed during the expansion phase.
__________________ Ren: Now listen, Cadet. I've got a job for you. See this button? Ren: Don't touch it! It's the History Eraser button, you fool! Stimpy: So what'll happen? Ren: That's just it. We don't know. Maybe something bad, maybe something good. I guess we'll never know, 'cause you're going to guard it. You won't touch it, will you?