If you've never had NPC sororities, it certainly doesn't hurt to try. Contact NPC for information about expansion and colonization. Go to the NPC web site.
You can also contact the non-NPC national sororities on this list and inquire about expansion with them:
http://en.wikipedia.org/wiki/List_of...and_sororities.
There are also non-collegiate sororities that you could invite to colonize. These are not specific to college students and could appeal to the city at large to include members from different schools or non-students:
http://www.greekchat.com/gcforums/sh...ad.php?t=54756
You can google this stuff. Just do some research and start contacting organizations. Please be warned that in many cases colonizations are a long process that won't be completed until well after you graduate. Getting a sorority started and off the ground is a considerable time commitment.