I did not suggest touching the existing ticket. But one PNR can contain more than one ticket. It is no technical problem and perfectly within the rules to add segments and to add tickets to an existing PNR. I do this very often. In the case of the OP I understand he has a ticketed PNR containing flights from MIA to SJO and back. My advice is to have an AA agent add flights from TLH to MIA and back to the PNR and issue a separate ticket for them. Then the PNR would contain all flights in the correct order (TLH-MIA-SJO-MIA-TLH) as well as one ticket MIA-SJO-MIA and another one TLH-MIA-TLH.

This way, every agent pulling up the PNR will immediately see the whole picture, which is much better than hoping that they read some cross-reference hidden in the OSI or remarks section (they probably won't). And the likeliness of getting automatically rebooked when the TLH-MIA flight is delayed is much higher.