on dx6i you also have the trainer switch, flap switch throttle hold button and flap switch, all can be used as on/off switches if you reprogramme the tx as below.
So if you have the elevator (right stick up and down) and this is redundant, but you want an on/off switch, you can use a mix to achieve this. You can see in the monitor screen what movement this is achieving, but as said previously, the best way is plug a servo into the receiver and see what is happening, whilst you get the on/off throw, it might need to be reversed.
go to adjust list in menu, go to mix 1
this will by default be inhibited. so to get the gear switch (or another) working on the elevator channel, you need to tell the transmitter this is what you want, so on the left scroll till you see gear, then scroll on the right to elev, and then "act" (activate) instead of inhibit.
sw (switch) needs to now be on, and scroll to the rates, you will need most likely zero percent on the left in menu, and then 100 % on right to achieve the movement. This might take some playing around, I think, if both are zero there would be no servo deflection, so in effect the switch isn't going from zero to 100% on
Back to list, and setup list, scroll down to monitor, flick the gear switch, and you will see the elevator indicator move, as you have now programmed the transmitter to do this, however, the gear channel is still active, you will see the indicator move with the elevator. Just go into the travel adjust and zero, in both directions, the gear channel.
Alternatively! (and I have done this)
open up the tx, disconnect the wires going to the elevator pot, fit a on/off switch to the case, somewhere that is convenient for you to use, and attach wires!)
It might sound a bit daunting if you have never played around with mixes, you cant break anything, so experiment. Ive just doen this with my dx6i for you, so I know this works. I do this sort of thing all the time, but you are limited to 6 channel with dx6i, I eventuyally bought a dx8 to get 8 channels, so I can use
throttle
ailerons
rudder
flaps
gear
elevator
aux1 for a sound unit
aux2 for lights
clockworks answer is spot on for just swopping elevator pot to a switch, but your tx has the capability of doing what you want if you experiment