Looks like only TAs can do this... But only if its 'live'. Which means you cannot add a segment booked on one reservation to add to another. They can only note that there is a booking as an info segment. So the preceeding airline may only see the booking...

Note that just because its in the booking does not necessarily mean that the airline will honor the interline agreement.

eg

BA 1234 LHR-FCO (booked elsewhere)
AA 1233 FCO-NYC

If the BA segment is added to AA's booking (in this case, anyone with Sabre/Abacus), AA can see BA's segment. But BA can't unless you add BA's segment with AA's booking info. But in any case, if BA sees that the segment is not 'live' they may still refuse to do the interline.

I am a TA with a GDS. What are your flights? Maybe i can look at it. Send me a PM.