I do not have access to the schematics for the transmitter but; from an engineering standpoint, this would not be a good idea. Power output is limited by law (at least in the USA) and this is usually done by the design using the voltage stated by the designer. Any increase in voltage will not increase the power as the voltage to the final is limited by the circuit design. All the excess voltage will do is dissipate unwanted heat and strain on the components used to regulate the power output. In short, the added cell will have adverse effect on reliability and life span. I'm sure if this is wrong, you will get a response from the manufacturer rep or other knowledgeable source.