Other than the old white 5,000 and 10,000 lights, all of our modifiers are interchangeable with all of our lights. So, anything that fits on an AlienBee will fit on your Ultra.
I do want to appologize, however, as most questions we get about grids are regarding softboxes. This is what I had in mind when I offered the suggestion of a DIY solution. I see now you did say reflectors specifically, which makes the suggestion not applicable.
We do have this item, and only one beam spread, that will fit over any of our 7" reflectors.
http://www.paulcbuff.com/gs1.php. I would suggest a new reflector or MainFrame and a selection of grids.