Whether or not you can mix this in your transmitter depends on the the transmitter. Some transmitters (typically entry level computer radios like the 662) don't include the trim input in user defined mixes. If that's the case, your elevator trim will only move the servo connected to the original elevator mix. Higher level transmitters will give you the option of including trim in your mixes.

A pre-defined mix like ailevator will include trim. So, if you have an ailevator mix, then you are all set. Ailevator is a mix that allows your elevators to act like ailerons as well. If you do have it, just set the aileron rate to 0.

An elevator to flap mix probably won't include trim since you wouldn't want your flap settings to change when you trim the elevator.

If you don't have the trim option in the mixes, the easist way is to use a reversing Y connector.