Issuing Custom Licenses for Extensions
For an extension that delivers a piece of software, which already has a proprietary licensing mechanism in place, it is possible to issue and use proprietary licenses while still handling license management through Plesk Key Administrator. This is done with the help of the ISV endpoint -- a custom web service that handles license creation, modification and termination requests for Key Administrator.
See Key Administrator ISV Licensing: Integration Guide for information on implementing the ISV endpoint, ISV Protocol reference, etc.
The workflow of issuing and installing a custom license for an extension can be found here.