I have a Futaba/Robbe FC16. The pots for the four gimballed sticks are 5k or thereabouts.
I wanted a rotary channel , and used a 1k pot with a 2k resistor on each outer leg; that gives you 5k, but enables one to sweep through the full range of the pot.
I was given a slider pot which is a robbe part, and that uses a 10k potentiometer with a 82k fixed reistor on the centre leg. Both work fine.
I can't say for sure if those values will work on your set, never having owned a 6EX, but I'd be surprised if they're radically different. Unfortunately circuit diagrams are difficult to find for Futaba kit, unless you know a tame service technician that works on them.