Call straight talk and ask them - that would be your best bet.
I have already done so, multiple times. I have been told it's possible but straight talk won't do it due to contractual stuff with att. In the mean time, I'm stuck with a $300+ piece of junk that misses more calls than it receives.
I know with Wind Mobile here in Canada, which is essentially the same thing as Straight Talk in the States, that the phone will automatically connect to different towers depending on location, even if you still have strong reception. This happens because they use multiple carriers towers and simply lease their use. By spreading the usage evenly between them, Wind ends up saving money on their lease.
There's an option in the Wind phones to set it to only search for a new access point if the connection goes below a certain value.
Is this option available with Straight Talk? You might have to take the phone to one of their retail outlets, or call them, and ask if it can be done.