Any way to select roaming network over home network?

I have 4 impressions on AT&T service. Where I live, we have horrible 1900mhz service. Recently, AT&T acquired Centennial, who has wonderful 850mhz service in our area. A blackberry on our account stays connected to Centennial all the time and works great everywhere, but the impressions stick with the bad AT&T service. Is there a way that I can change the network selection or lock the phones into roaming only? I will not incur any fees since the 2 networks are technically merging soon and both owned by AT&T. I just want decent in building coverage with the impressions as well. Thanks in advance for all your help!