Can prior service join the air force?

Yes they can. I just joined the Air Force Reserve and I am prior service in the Army. It is hard to get into the AF reserves or active duty if you are prior service, because the recruiter has to do more work. Some recruiters do not want to go through the effort when they typically have more people wanting to join than positions that are open. However, it is possible and, if you really want to do it, keep trying until you find a recruiter who is willing to help you.