Your hi-hat legs are fixed but you'd like to use a double pedal. You may be able to. Here's what I did with my Yamaha hi-hat stand:
Removed the allen screw at the base of the tripod above the pedal
Replaced with an old hi-hat clutch wing screw. (you might have to experiment with the size)
Added a hose clamp above so the legs would not move up and down when loosened. Loosen, swivel the pedal over, re-tighten and you can now fit your double bass slave pedal.