There are many national/regional social sororities you could talk to if you want to affiliate with a social sorority, thus become national on campus and stay women-only. See the following thread on non-NPC social sororities:
http://www.greekchat.com/gcforums/sh...ad.php?t=72960
If you have a time and the university allows it, the other possibility is to set your sorority up to become a regional/national sorority and expand to other universities. This would take a lot of commitment from your alumnae to help set up a national board, incorporate the organization, obtain insurance, etc.