The Nord Electro 6 has a control pedal input so if you have a control pedal you can use it to modulate everything as you can with the modwheel. Maybe you have to activate control change in the NE 6 system menu (I only have a Nord Stage 2 + 3). The connected pedal will then send out CC 11 (Expression) messages. You either configure Omnisphere to react to CC 11 or you use the GP MIDI Filter to map CC 11 to CC 7 (Modwheel) so Omnisphere will never know you don’t have a modulation wheel 
