You can easily do this so long as your agent uses UAL's Apollo reservation system. You don't need to go to the CTO, just tell your agent the PNR.

I used to do this all the time, first with Amex/Travel One, then with UA's United Connection Travel Desk Pilot Project (which I really miss). I now can't do this anymore as Amex/One has gone to LH's Amadeus system, and they must book my tickets for our contract rates. But I still use the windows product to determine itineraries, then tell the agent the flight #s...