wishinhopin,
It looks as if your next move is to notify Student Activities that you are interested in getting someone from administration appointed to oversee the extension process.
Then get your core group of girls who weren't offered a bid ready (maybe a mass email) and let them know that you're calling a meeting about the NPC extension process. Make that meeting informational/organizational, so that everyone knows what the process is. You can start getting your constitution and committees in order. I would suggest having all your ducks in a row before advertising on the general campus.
Hope this helps, and if anyone can add on this, please do so! Let's help Corina out!
(PS: I know that you already have one particular sorority in mind, but I would urge you to let NPC notify all groups of the extension opportunity. IMO, its best to keep your options as wide open as possible. You never know what will happen!)