It should be the same procedure to merge the AIR SDK with Adobe Flex or Apache Flex. Two things to watch out for, that I remember:
1) Make sure that you get the AIR SDK for Flex developers, and not the standalone AIR SDK that has its own compiler. The Flex SDK already has a compiler, so mixing both will cause issues.
2) If you're on macOS, copying everything over in Finder will replace subdirectories instead of merging them, which will result in missing files. There's a particular terminal command that is recommended for merging two directories, but I don't recall it offhand.
One more thing: Apache Flex requires an extra installation step, so you can't just download a .zip or .tar.gz file and extract it. There's an installer.xml Ant script that needs to be run. This used to be easy to handle by using the Flex SDK Installer desktop app, which would run the script automatically after downloading, but I'm not sure how well that app works anymore. Anyway, after the installer script is run, then it should be possible to merge HARMAN AIR.