In KiCad V5.1 you can select a pad and some graphics and then select: “Create pad from selected shapes”.
I can not figure out how to do something equivalent in V5.99.
When you edit the properties of a pad in V5.99 there is a tab with “custom shape primitives” but all options are greyed out.
Maybe it has something to do with the “grouping” function, but that does not seem to be finished yet.
Right click pad -> Edit Pad as Graphic Shapes (Ctrl + E). You can then right-click again and Finish Pad Edit (Ctrl + E).
I haven’t played with this too much but I think the idea is you add a numbered pad, “edit as graphic shapes”, draw whatever shapes you want, and then when you “Finish Pad Edit” anything that’s touching your original pad becomes part of that pad.