It can be done. Do a search for the flight you want. Select that flight and don't forget to tick first or economy. Click "Select Flight". The flight selected should appear. Click "add more flights". This will allow listing under the same PNR.
NO, what you need to do, is when you search for first flight, search one-way. after selecting flts it will ask you if you are done or would you like to add more, the select add more and add more one-way tickets.
