Even if it is possible, sprint more than frowns upon excessive roaming, they can even cancel your service. There is a percentage allowed for roaming. I want to say 50% of the time but it may be less.
When I dumped AT&T and was looking for a new provider, I ran into the same problems. I live in the middle of nowhere and I was looking at C-Spire which was Cellular South. They had excellent coverage in the Memphis area, but the further out I went, the more I was roaming. The sales woman told me the same thing - if you spend most of your time roaming, they can cancel your service and slap you with an ETF.