I know how to do it on a Spektrum DX8, but that's probably not going to help you. Essentially, all you have to do is to take the input from a three position switch and feed it into the ailerons. You will have to reverse one of the mixes. I would start with about 15 degrees of deflection. You can choose to have spoileron-neutral-flaperon or neutral-light flap-heavy flap depending how you set the offset of the mixes. You may also want to mix in some elevator. When you're in the flaperon position, mix to feed in an equal amount of down elevator. The opposite (up elevator) for spoilerons. You'll have to experiment to find an elevator mix that makes it fly neutral.
I would have asked you to come to my field so I could help you set it up, but it doesn't look like I'll have time for any flying this weekend. Anyway, you have a standing invitation to any of my clubs!