Hey Matt,

You're going to need to make a custom fixture XML file and put it in the FixturesType.XML inside pixelMAD.

The bigger question is how are you planning on controlling the rest of the non-RGB parameters in the fixture (Pan, Tilt, Contol, etc)?
Are you going to do some sort fo DMX Merge between Catalyst and your Desk?

Depending on how this happens you may find you need to set the "unused" or "desk controlled" parameters to either 0, 128, or 255 depending on how the Merge is structured (HTP vs LTP for example).

Hope this helps.