Is there a way to increase the voltage to only one accessory while keeping it constant to the others? I do not want to add a separate transformer for only one accessory
This is the wrong forum to ask a question, and I suppose you could just be testing how to post and used your question as the text post, but to answer your question...
You could set the power supply to the highest voltage you want for any of the accessories and then put a resistor in series with each device that you want to have a lower voltage. The resistance value of the series resistor will depend on the current that the device will draw. Some devices may require a up to 3 resistors to make a voltage divider to get what you want, and again the values of the resistors will depend on the current required by the device(s). And some devices may require a bit more complicated circuitry to maintain the voltage/current at acceptable levels.
Seems an awful lot of trouble to go through to save adding a separate supply for one accessory.