Short answer: Probably not.
Long answer: The reason isn't technical, it's economic.
Most phones are sold by the service providers for less than they actually cost. The cellular phone companies aren't in the business of selling phones, they're in the business of selling air time. But since customers can't use air time without a phone, the service providers want to get as many phones out into circulation as possible. To that end, they subsidize them.
It's like Polaroid cameras. When you buy one of the low cost cameras, you're actually paying less than it costs Polaroid to make and sell it. That's because if you own a camera, you'll be buying film for it. Only Polaroid makes film for those cameras, and that's where they make their money off of you. By taking a one-time loss on the camera, they hope to make a continuing profit off the film.
Now of course, that's not necessarily going to happen for every single camera they sell. But statistically speaking, the more cameras there are out there, the more film they'll sell.
The service providers do the same thing. But while only Polaroid sells film for their cameras, all the CDMA phones are compatible with all the CDMA phone systems in the same frequency band (generally speaking).
If provider XYZ sells you a phone at a loss, they do so in expectation that statistically speaking you'll pay as much or more in air-time fees as they lost on the phone. For a one-time loss on the phone they expect to make continuing profit off of your airtime use.
But that's just what won't happen if you take that phone to some other service provider. In that case, XYZ is out the subsidy and gets nothing in return. Needless to say, they take a dim view of this prospect.
So the phone manufacturers have implemented what are known as subsidy locks. What that means is that the phones are inhibited so that a phone sold by XYZ won't work with carrier ABC, even though ABC sells exactly the same model. Usually this is done by locking the phone's NAM, so that it can't be reprogrammed to use a different service provider as the home system. (This does not prevent you from roaming to the other system.)
If the phone has a subsidy lock on it, and most do, then only the provider which originally sold the phone can lift the lock. You can ask, but they probably won't do so for the simple reason that there is absolutely no incentive for them to. It doesn't make for customer good-will, since you're trying to be someone else's customer anyway. It merely makes it easier and more attractive for you to leave, which clearly they don't want. After all, if you have to buy a new phone to switch service providers, you're less likely to do so.