I use the Sprint Branded PPC6700 on Cricket Wireless, and it wasn't easy getting everything working properly.
The most important part is to get the Cricket PRL installed on the phone.
Download the Cricket PRL. (Google search)
From there you need to move that file to a folder on your phone via. PC. I just put it in the Documents folder, it was continent. You don't have to use a data cable, you could put it on a storage card and use it from there. Loading the PRL file on the other hand is going to be something totally different. For sprint, which isn't very well documented because most people use the XV6700 from Verizon, Verizons SPC code (000000) is pretty well known and easily found if changed, Sprints MSL code isn't something published and is different on every phone. I found on the PPC-6700 from Sprint the MSL had something to do with the ESN of the handset and I found a program that'll convert the HEX ESN into the MSL code, which is required to make Provisioning changes to the phones. This utility doesn't seem to work with the other sprint phones I've tried.
You'll have to figure out what its going to take for the AllTEL phone.
Sprint you go into the phone mode and type ##775#SEND to access Manual PRL update. This is where you need teh MSL code. ##778#SEND gets you into the Provisioning menus of the Sprint model, which you can view everything except Security settings without MSL code, but to edit anything requires the code.
I guess give those two programming sequences out and see if they're anything similar, but you'll likely have to crack it yourself or find someone who has successfully converted an AllTel model.
There are several forums on the internet for Cricket conversion assistance and other hacking and I'll list the two I visit most. All I can say is Search extensively using the sites search feature BEFORE you post a new topic asking for help.
http://www.howardforums.com/forumdisplay.php?f=336
http://cricketmafia.freeforums.org/
As of right now the only features that work on my PPC6700 on Cricket are Voice, Web browsing, and Text messages. MMS messaging isn't something that seems supported on this handset with Sprints ROM. I'm not brave enough to try and flash a new ROM onto the phone so I'll likely just not deal with MMS (picture messaging) at all.
Good luck!