Mortising Drill Bit Set-Up?
Simon Watts: Mortising bits cut a round hole. They are used inside a hollow chisel that makes the hole square. So the answer is, yes, you need the whole setup. You can, of course, drill out the waste and cut the mortises by hand with a chisel.
Michael Dresdner: A mortising set up consists of a drill bit inside a square, hollow chisel. The bit inside spins, the chisel does not - it merely pushes down through the wood as the hole is cut. A mortising setup arranges this. Some are made to retrofit to a standard drill press, but clearly, it is more than just a drill bit.
Rob Johnstone: There are aftermarket mortising tools available for almost any full size drill press. You will need the whole setup, not just the chisel and auger.
Ellis Walentine: Some drill press mortising attachments are available, but they aren't generally well regarded for the quality of their results or their convenience. If hollow chisel mortising is something you will be doing fairly regularly, I'd suggest a dedicated machine rather than a drill press conversion.
On another note, I'm kind of a snob about these tiny hollow chisel mortisers that have flooded the market in the past decade. The best of them come with decent quality chisels and parts that actually fit together properly, but they all suffer the same limitations of depth and width of mortise. As an alternative, consider making yourself a great mortising jig for your plunge router and shift over to loose tenon joinery.