Just bumping this in case anyone has found a way of adding a PC number to an existing paid reservation. I tried to modify a res (when not logged in), then logging in, reverting to the country from which the rate was paid (also skipped this step), but the reservation cannot be modified in any way. At the second screen the site throws a

"CHANGE REQUEST CANNOT BE ACTIONED- MUST CANCEL AND MAKE NEW RES [DE685]"

or earlier if one picks a different car or a different time/date.

The procedure above works just fine for pay at location reservations.