You aren't just able to "tell" an antenna to use another frequency...

Antennas are a length of wire of a certain length to allow for it to resonate with a particular frequency.
These antennas can operate over multiple frequencies *if* the other frequency is a harmonic of your base frequency.

T-Mobile uses 1700MHz and 2100 MHz for their 3g network.
AT&T uses 1900MHz, as do the other 3G networks around the world.

While it is *possible* that the antenna will have harmonics visible in both the 1700MHz and 2100MHz band, it is unlikely that one would be able to receive any significant amperage or broadcast across that frequency.

Someone would need to do some tests to see whether this is viable, however from a technical perspective, it is *very* unlikely that you would end up with any meaningful coverage area using the iPhone 3GS on T-Mobile's 3G network.

~Adrian