hmorgan883 welcome to the QR forum. Look around and you will find a wealth of useful information. Use the dashboard to get you started.

Yor question was raised quite recently and no answer was forthcoming.
I asked a local TA about adding feeder flights to a reservation and he said: "A TA can do it but it is difficult once a PNR has been issued. In Theory OTAs can do it but there systems are not geared to do so. Airlines have the power to do it but they normally refuse to do so or demand a complete ticket reissue. If it is not at least a partner airline then it is 99.99% certain that they will refuse.". I am paraphrasing a ten minute explanation, some of which I did not understand, so I have probably got something wrong. But that is the best answer I have.