Well........This is sort of possible but it is a bit "rough" I would call it. This would be the only way to do this really.
Make 4 switches, 2 with the 'add' priority and 2 with the 'sub' property and call them "X + offset", "Y + offset", "X – offset", "Y – offset" then use the dimmer fader on these switches to set an offset.
I know we don't have anything built into Compu Live like a Hog does. But for our newest program Compu Show we have it on a "to do" list that is ever growing. Which model interface do you have?
Sincerely,