Thank you for that, though after further reading, Extension Packages only appears in WATO for Enterprise Editions.
That did get me on the right track to find what I needed in the documentation, though:
To install a package in the Checkmk Raw Edition you can use the command line as usual. You can find the MKPs under
~share/check_mk/optional_packages/.
(from Checkmk extension packages (MKPs) - Integrating extensions from the community into Checkmk)