Yes you can convert the alltel phone. Most people use verizon because they're the easiest... Verizon doesn't lock their phones... However, you can use a sprint phone or an alltel phone. I know sprint locks their phones, so you'll either need to security code or an unlocker utility... I'm not sure if Alltel locks theirs or not.
I've heard that Alltell doesn't put any of the customized BS on their phones, so in some ways, it might be a better choice..