You're right, you
could raise a panel with a grooving cutter. But there's plenty of reasons why you
wouldn't.
Firstly the panel that's produced isn't the best. I'll illustrate the point with a sketch.
The top panel is what's produced with a dedicated panel raising cutter. As you can see there's a relatively large, flat area that engages with the groove in the rails and stiles. The bottom panel is what you'll get by using a grooving cutter. Given that you want the panel to be a snug fit you can see that with a grooving cutter it has to be very precisely sized, too thin and it's a sloppy fit which will rattle every time the door's opened or closed, too fat and it will force the rails and stiles apart preventing you from getting good tight mortice and tenon joints.
Secondly the work itself is a bit dodgy, there's a big gap in the fence with all that large diameter tooling and possibly a big gap in the bed of the spindle moulder around the spindle shaft. Either of these is an invitation for the workpiece to get sucked in, chewed up, and spat back out in your direction as splinters. Also, I previously mentioned the risk of the "falling piece", which means (depending on the dimensions of your grooving cutter) you'll probably need multiple incremental passes.
Thirdly, how many doors are you making? The reason I ask is that I'd allow an hour's set up time for this job, but I've been around spindle moulders for forty years, if you're new to it you may well spend a day on the set up alone and need multiple test pieces before you're good to go. For a start you'll need a sled (yes, you could make the cuts on the underside of the workpiece, but I wouldn't because you're just asking for variations in workpiece thickness and the panel is inherently less stable on the bed done that way), the sled will need a piece of timber fastening on perpendicular to the fence to serve as a break out guard, and then you'll need a couple of toggle cramps fastened onto this to hold the workpiece. I could spend twenty minutes just talking about safe and unsafe practises in attaching toggle cramps...but life's too short! The sled must be long enough to span the opening in the fence, ideally clearing the fence gap both before the cutter engages and after it completes the cut. You'll also need a false fence, but you can't just push that back into the spinning cutter to make a zero clearance fit, as a tilted cutter won't cut through the fence (if you can't envisage why not then take it from me, you're not ready to do this procedure), so there's another half an hour with a jig saw right there.
The list goes on and on.
Doing this right is not a quick and simple job. It's possible, but it needs a lot of thinking through. Spindle moulders aren't like other woodworking machinery in two key respects, firstly the tooling and accessories nearly always cost more than the machine itself, and secondly each new piece of tooling transforms it into effectively a new and different machine with its own set of safety requirements and operating procedures.
Invest in face to face training, it's the only sensible way to operate a spindle moulder.
