Depends on your defintion of "open" segments:

On a RTW itinerary, you can fly into Detroit and then jump on the next plane in Chicago... NWA will put the mileage between Detroit and Chicago into the RTW record as an "open jaw" segment, even though you're not flying.

If you're talking about the other definition, where you plot out only the first flight and any of the others are undefined... I don't think so But even if you could, they'd gleefully charge you the $150 fee to make any changes to the itinerary after its ticketed.