Yes, you can bring the romex into the driver box and put a little romex connector in one of the knockouts. You'll need some slack so that you can slide the panel in and out of the frame.
CD
Sorry Smalltown, I thought there was some reason you were running conduit. The way cyberdyke describes it above Is how I would plan to do it with Romex. I am curious though how much room is in that box? What are the dimensions, length width depth? If you are connecting more than one light on a circuit I am concerned that little box can fill up pretty quick. It might be plenty big, I don't know
On my project my trusses are the room over type and I want to insulate the first floor ceiling and have the upstairs as cold storage. So to do this I am planning to run romex into ceiling/round boxes where there is plenty of room to make connections, then insulate then drywall. This may only need to be done in the area where there is a room above, only about half the garage. I will then drop out of the round box with a knockout cover and one run of romex to the driver box. The low voltage wire for dimmer does not have to be protected from what I have read. I plan to push that through directly beside the round box in the ceiling and into the driver box. If the driver box turns out to be large enough to accommodate two romex coming into the driver box and connections made there I may opt to not install boxes in the rest of my area and just push them through the drywall.
Thanks