I have been able from time to time to get google flights to spit out RTW fares and it has transferred that over to united.com. Now, depending on how complicated your RTW is, that route won't work.
You have to know what your want and need and then an agent with the help of the rate desk can book RTW, I have been successful on the phone.
You might get more PQP and RDM if you book on non-UA ticket stock since you'll be able to get around the 75k limit. Also, important to check which booking classes these book into depending on where you buy it and so on. Some UA Z will map to LH D for example on RtW, which is also then upgradable with +Ps. So take your time to figure all of these things out before committing to where to buy what ... can make a pretty substantive difference in the end.
A really good TA (some here on FT) might be able to help you with figuring out the best way.