When drawing a polygon you can use arcs (just like when tracking) by switching drawing modes using SHIFT+SPACE. If needing a circular cutout then I would suggest simply drawing a circle and converting to a cutout. Draw a shape (using lines, arcs, circles etc) and then in the search box type "conv reg" to activate the command to convert a set of drawing primitives into a region.
Once a region is created bring up the property dialog (double click or right click > Properties) and change to be a board cutout.
Regarding STEP output, very little is configurable so I don't think there is a way to output random layers and objects.