I'll assume you're referring to a feature of the third-party VAICOM PRO addon, as "TX1" is not a term VoiceAttack uses natively.
A single command can be triggered using a single button (combination; combination meaning multiple buttons need to be pressed at the same time).
However, you can create a new command, set it up to be triggered by the other button (combination) you want to use, and have that command execute the original command using the "Execute Another Command" action.
In the majority of circumstances, that should be a feasible approach.