Can UA agent book non-UA operated flights?

I am interested to book an itinerary XXX-YYY-ZZZ. UA published a fare for XXX-ZZZ market with routing XXX-UA-YYY-**-ZZZ, where XXX-YYY is operated by UA and YYY-ZZZ is operated by a non-UA airline. UA does not even have codeshare with ** airline for YYY-ZZZ market. However, it is a legal routing permitted by the XXX-ZZZ fare and I can price it correctly on ITA.

Can I book it on united.com or UA phone agent for this itinerary involving non-UA coded/operated flights? If not, what's my best OTA choice for the itinerary (a complicated one, 8-10 segments in total for the roundtrip)?

Thanks.