Hi,
There are a number of ways to accomplish what you want.
Method 1:
The following, in my opinion, might be the simplest, but uses the left slider instead of right. I'll assume that by "side trim" you're referring to the sliders on the side of the radio.
1) Leave your ail servos on matchboxes.
2) Select code 22 (wing type) in the radio.
3) Choose the "Flaperon" option for wing type.
4) Plug right ail lead into the ail slot on the rx. Plug the left ail lead into the aux1/flap slot on the rx.
5) Select code 17 "Function select."
6) Inhibit any functions that you are not using - make sure you leave the flap function "active" if you want to use your ail. as flaps.

These steps allow you to use the left slider as the flap lever.

t