I assume its a PS/2 connector based keyboard. Some USB devices can be stepped down to PS/2, but it doesn’t go the other way. In order to do that you need a usb controller; I think they make ps/2 to usb adapters, but have no idea what they cost. You might be able to jury rig something; I’ll look into it because I’m curious.
Do you just want an external keyboard or does the origanal not function properly? Because I do know off hand of a way to replace most laptop keyboards, including those of the TR2A, with a standard ps/2 keyboard; it would likely be messy as hell on the TR2A though.