From Terms and Conditions:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">One-Way travel, stopovers, circle trips, open segments, open jaw and waitlisting are not permitted</font>
That implies that you cannot do what you are planning to. Also since you must book your websaver online and their web booking system does not support open jaw bookings, there's no way you can do it.