What you could do is go into the drum key config in DTXMania and swap out the commands. There should be a bunch of assigned keys, so if you want to find it it's just trial and error. =/
Now, if you didn't want to use the hihat pedal at all, what you could do is just make it so the hihat works for both open and closed regardless of pedal by doing:
Under configuration--->System:
HHGroup: HH3
Ride/Crash Cymbal: CYGroup: CY1
Floortom when you hit red: FTGroup: FT1
^ Cymbal and floortom as well if you don't want to bother with those |