If I understand correctly, currently the App Store Connect API for in-app purchases is read-only. I guess it may gain features to add/modify IAPs eventually.
In the meantime, the older XML "Transporter" tool still exists. I don't think this is actually deprecated, but the documentation is very difficult to find. I believe it is mainly used by people who upload music and video to the iTunes store. If your app has a catalogue of dozens or hundreds of IAPs, Transporter is the best tool.
Note that it is Java, so you can run it on your Linux cloud instance if that is useful to you.
See WWDC 2013, session 306:
https://developer.apple.com/videos/play/wwdc2013/306/That refers to a WWDC 2012 presentation.
