Is your one way full Y (or refunable)? If so, it may cost the same amount to just add the leg to your existing ticket.

Example:

Last year to take advantage of the 55K, I bought a one way LAX-MAD. Routing was LAX-YYX-ORD-LHR-BRU-MAN-FRA-MAD. All at a faily low Y fare.

So perhaps you can be creative, and either add your segment to the outbound or to the return.