If you're trying to keep your letters etc. it probably won't happen if you're trying to go national at the same time. However, you can always keep your traditions and symbols as chapter and local traditions while being a part of a bigger organization.
It sounds like your campus is open to expansion so your biggest hurdle is over. Every woman here who belongs to a national sorority can go on for hours about the benefits of being in a national (or international!) organization. I would seriously suggest going through the NPC. Try to become an NPC group. You don't have to lose everything when you go national.
http://www.npcwomen.org
