Make OSX recognize a folder extension as a package

I have an aax plugin which is able to export a session like folder structure. This folder has its own extension and contains wav and xml files. How can I make OSX recognize this new extension in order to treat this folder like a package (like an aaxplugin itself)?

Not sure how to do this when creating a file by code, but via script you can mark a specific file as a bundle using SetFile:

/usr/bin/SetFile -a B myfile