I'm not positive but, you have to set up a mix so that channel 5 parallels channel 6. You also have to do a mix to cancel out the switch that normally operates channel 5. I'd do the mix to null the switch first so you can verify it has no effect then do the mix to parallel channel 5 and 6. Then when you plug your left aileron into channel 5, it behaves the same as if it were channel 6. The only thing I'm not sure of is if the aileron trim will affect channel 5. I did this several years ago when I had the same situation. I have long since got rid of my 6XS radio or I'd verify how it's done.
basmntdweller