The website is indeed frustrating. I was looking to book a mixed class itin with a long voluntary overnight transit in HKG. However, the RT option won't show the early enough flight (just an hour earlier, but with availability in PY) and the multicity won't allow a mixed itin.

Booked and called to upgauge, but they quoted a ridiculous upcharge similar to buying a J RT outright. I just gave up.

ETA: Transit, as time on ground is well under 24 hrs.