what i believe is being asked here - is -

when in preset mode ( not running cues from DMX ) - is there a way to control JUST the intensity from a lighting console fader?

And there isnt - it would be hard for me to do this-

It would need a single channel fixture.

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Such a thing is possible with the 200$ Behringer BCF 2000 - flying fader midi controller.

You can control the intensity using the faders on this - and the best thing is that the flying faders are in sync with the intensity values that might be stored in any preset.