Hill has followed Barton and Kranjcar to Rangers
Hill became the third former R's player to move to Glasgow this summer, after Joey Barton and Nico Kranjcar, having signed a one-year contract last month.
And Onyewu, who spent spent two months at Loftus Road during the 2013-14 season, could make it four if he impresses on trial.
The American has been without a club since leaving Charlton Athletic in a year ago and recently linked up with the Gers during their summer trip to the States.
Onyewu (left) joined Sheffield Wednesday after leaving Loftus Road
Manager Mark Warburton is known to be seeking a fourth centre-half, adding to Rob Kiernan, Danny Wilson and Hill, and the latter believes Onyewu can still do a job - even at the age of 34.
"It’s like a QPR reunion!" Hill told the Scottish Sun. "He was with us for a few months but he struggled with injuries during that time.
"Hopefully he can put in a good shift and make an impression on the gaffer.
"Look at his CV - Newcastle, AC Milan, Sheffield Wednesday, who are another big club.
Onyewu (left) spent 18 months in Milan
"He has masses of experience and it’s up to him to grab it with both hands and try to get a contract."
Onyewu, who Zlatan Ibrahimovic claims once broke his ribs in a fight, has also turned out for Sporting CP, Standard Liege and Malaga during a 14-year professional career.
He has been capped 69 times by the USA, for whom his last appearance came two years ago.