You can define 2D regions then click the "Board Cutout" checkbox to create cutout regions. This makes square cutouts, but you can then click on the region, place the cursor over a corner, then while holding down "Ctrl-A", drag the corner inward. That will create a rounded corner. You can drag it inward to increase the radius, so if you start with a square with edge length of the intended diameter, then change the grid so a grid point is on the center of each edge, then you can turn the square into a circle. Then click "Board Cutout" in the properties dialog. I searched for a way to create a region from primitives or polygons, and couldn't find it.
I expected the Board Shape <-> primitives tools to include cutouts, but I tried and couldn't make that happen. Perhaps I missed something.
The easiest way is to have a mechanical guy do the board, including cutouts, in a MCAD program and export a STEP file, then create the board from the STEP. That's what I do when I can. If you don't have a mechanical guru working with you, but you are decent with an MCAD program and have one available, I'd say it's still the best way.