PCells can be created from shapes if the PCell is derived from a "guiding shape". Specifically the Basic library PCells support derivation from guiding shapes with a few exceptions. So for example, a round-cornered polygon can be created from a normal polygon by selecting the polygons, choosing "Edit/Selection/Convert To PCell" and selecting the "Basic.ROUND_POLYGON" for the PCell.
PCells can be converted to normal cells by choosing "Edit/Cell/Convert Cell To Static" or "Edit/Layout/Convert All Cells To Static". Normal (static) cells can be edited individually but do no longer offer parameters to control the look of the cell.