You need to sign up for a cricket account, then call cricket and have them port the nuraber.. it should take a day or so (this might be different now, it was like 4 years ago that I ported mine... on the first week that porting started)...
It was a pretty painless process. you'll have a different nuraber with cricket until they port your nuraber from alltel, but like I said, that should take less than a day (I'm assuming it's almost instant now).