Well, although I'm not NPC...I think that there are a *few* that will allow graduate students to have collegiate standing, but you'd probably want to check the various national websites and you can get that from
http://www3.telus.net/greek/.
however, you might want to look into Alumnae Initiation if the rushing route is closed to you and you're still interested, although it is a lot of work (but worth it!).
If there are local sororities, you mighthave to approach them on your own accord rather than through rush, as their rules of initiation and membership are probably different from the NPC sororities. I believe my local sorority has initiated graduate students before, but because our school doesnt have a huge graduate program, it's not "regular". We've had several older initiates in organizations here...one fraternity initiated 9 guys last year with an average age of 25. Most of them were exchange students and one was just an old fart coming back to get another degree (he's really nice though!).
Good luck!