It is definitely possible and some shops will do it.
It does take more specification than a normal hole because Gerbers are just 2 dimensional, but modern PCBs are really 3 dimensional products. The removed material doesn't create any more issues than any other milling operation on the PCB, such as slots or even regular drilled holes..
Treat your countersunk hole as any other part with a single hole. The diameter should be the thru hole diameter you need for your screws. The hole should be unplated and no pads on any layer. Now this will give you the placement and FAB accuracy you need. Specify your countersink details only on the fab drawing using the same method you would for solid metal or sheet metal part. This include countersink angle and the depth of the countersink operation.
As an aside, why do you want or need countersunk holes? If you're hoping for more accurate board alignment, you may see that OR, the first screw that's tightened down will lock an X/Y coordinate. The other holes will be trying to pull the board in their direction. For a frightening 50 years of product development, I've never used other than pan/binding head screws and helical lock washer if needed, or snap in plastic fasteners for lower cost products. Or in some cases, mounting holes may fit over injection molded "pins" in your design, eliminating hardware.
But perhaps you have a need for flushness on one side of the assembly.
If the question was directed it me, I don't need countersunk holes I just thought it was an interesting original question as I hadn't come across any specific means to do so in Kicad - the only PCB app I've used.
HAHA. Thanks for your answer. I liked the last part. The answer is some vendor sent us a CAD drawing of their board and it had countersink holes (I never saw it - I only have the actual board). Our Mechanical engineers made design decisions based on that, in two different places. I designed a complimentary board for the other location. Then when I gave them the design and the vendor part they were like.. uhhh.... We need these be countersink. I told them I didn't think that was going to work and I would let them know.
HAHA. Thanks for your answer. I liked the last part. The answer is some vendor sent us a CAD drawing of their board and it had countersink holes (I never saw it - I only have the actual board). Our Mechanical engineers made design decisions based on that, in two different places. I designed a complimentary board for the other location. Then when I gave them the design and the vendor part they were like.. uhhh.... We need these be countersink. I told them I didn't think that was going to work and I would let them know.
