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.