Have you actually done this? Are there specific things to tell the agent to do to make this work? A few (very few) people have indicated that this "can" happen, but no one has been able to come forth with specifics, unfortunately. A vastly larger number of people have been told that it can't be done.
Cheers.
In my airline days as a supervisor I did this a few times. I would then have to call ticketing and have it ticketed manually. The key in the PNR was to put an ARUNK between the award and paid segments, even if it was not necessary. An agent would have to be familiar with native SABRE today to do this.