KiCAD is a 2d tool. The only way to specify a conical hole generated with a conical bit is in the tool designation for drilled holes. There are no drawing features that would help to show the hole design more clearly.
The manufacturer might be asking for a 3D design or a much clearer description of exactly what is being requested. If you need 3D design, perhaps FreeCAD software could help.
Or maybe they are just asking for the 2d/layer design around it. It which case, just draw a simple representative design, and add a mechanical hole of the right surface radius. Leave as much clearance as you can since the manufacturer will be dealing with depth (Z) variation, in addition to X and Y variation. Make sure the size you use is unique, and specify that the hole of that size in the design is the conical hole.
If they are making the hole with a conical bit and vertical CNC, then your choice of angle will be limited to the bits available to the manufacturer. Hopefully you want all holes to be the same angle. If not, you'll need to specify each as a different tool (which I think from KiCAD can be done as different sizes).