I read the OPs question entirely differently. They want to but a revenue ticket on some Star Alliance airline in a booking class high enough for Star Alliance upgrades then upgrade said ticket with UA miles. They'd be crediting the flight to SA Voyager. Frankly I've used UA miles just 2 days ago to upgrade an NH flight and I don't see any reason it couldn't be done. Worse case scenario, put the UA number in the reservation, upgeade, then change to Voyager number at check in but I don't even this this is necessary as you can access the ticket info with PNR and ticket number. The uograde is confirmed with X carrier on the spot. UA isn't going to check back an hour before the flight to make sure you're actually accruing to a UA account.