It doesn't appear that Design Line can handle this situation - unless it is possible using custom programming. In any event, most people who use the software don't have the time to deal with such customization.
I manually placed an NHCI upright wye along with a couple of pipes in/out of the fitting and used REVDESIGN to create a Design Line (see A). I then copied the DL and used 3D fill but the software could not correctly produce/place the upright wye fitting (see B).
I then tried manually placing an upright wye and add Design Lines to the fitting end connections (see C) - pipe was added but no couplings. I could have added couplings but isn't that Design Line's job?
Apparently an upright wye fitting would be treated like a wye with 8th bend but since there is no vertex where the wye branch and 8th bend would meet, populating with Design Line fails but a wye and 8th bend fills correctly (see D).
Makes you think twice about using upright wyes with Design Lines....