Absolutely, at least, on Star Alliance award reservations. I just did it recently. I have a PDX to SIN award on SQ. I decided to make SIN as a stopover (3 days) upon return from MNL. With F seats available on SIN-MNL segment available, it was done in few seconds. On my case, since I made a change on a previously ticketed award reservation, I had to pay $100 plus change in tax (fee however was waived at PDX to my surprise). So yes, you should take advantage of it, I would suggest, if you can.