You are suggesting something so completely irresponsible it's difficult to justify the time I have already spent on this.
Someone correct me if my understanding is mistaken, but unused transponders are off. There is a closed loop around repeating on a transponder which involves telemetry and command uplinks to the satellite, interactively set and monitored by the satellite owner. You simply won't be able to do it.
You will likely be off focus, or your beamwidth will be too wide, or your cross-pol will be horrible, or your harmonics will be violating emission standards. Even worse, any combination of the above. You will interfere with something. At which point, you'll be liable to prison time and multiple thousands of dollars in fines from the FCC.