UA offers a pay up either at the check in kiosk at the airport or if you check in online 24 hrs before your flight. A couple of weeks ago, I flew from SFO-PEK on UA. I had a (T) economy ticket. When I was checking in online, UA offered a $1,300 upgrade to J or 30K miles plus $600. I took the 30K +$600.

One thing though, you will not get the extra miles to your UA account as the miles will be on the original ticket purchased.