Not sure on the particular model of phone, but it is likely that T-Mobile had custom firmware made for the phone. While firmware is upgradable, it's very difficult to do without a data cable, and very risky even with a data cable.
I'd say you're probably out of luck and have to do it manually, unless you are very good at hex editing and custom modifications to code.
That said, the USB cable may work, with the right software could load the phone into a bootloader upon startup (before t-mo's customizations kick in), and then you could customize to remove the bootloader. I have no idea if such a program exists for that particular phone though, and by the time you do all that you might as well have copied it by hand.