Helix/HX is fully compatible with the Line 6 EX-1 and Mission Engineering's EP1-L6 external expression pedals. Other pedals may work with Helix, although they may not be fully compatible or tested, such as the Dunlop DVP1XL.
In some cases, the pedals internal dip switches may need to be adjusted to ensure proper polarity. If you experience trouble with inaccurate expression pedal swing, try adjusting any internal or external dip switches the pedal may have.
Also note that any expression pedals that appear to work backwards can be set to work properly by setting Global Settings > Switches/Pedals > EXP 2 Polarity to “Inverted.”