Reply to Set FX on when modifer state changes (Map FX to Fader)

I'm not sure if this is possible but I'm trying to map a fader on my Kontrol F1 to turn on an FX unit.
What i'm trying to achieve is as I push the fader up the FX unit turns on and changes parameters, as I slam it down it turns off. I'd like to avoid using a button to turn on the FX unit.

I've tried mapping
FX Unit On -> Volume Fader 3 however I get an error "invalid control"

However I can set Volume Fader 3 to change a modifier
At the moment with the fader at 0 Modifier 6 = 0
Fader at anything else Modifier 6 = 1

So is there some trickery I can use here?

ie
if modifier6=1 then FxUnit3=on else FxUnit3=off