Hello John,
Thanks for reaching out to us with your question!
Yes, it is possible to do what you described. In this case you will have to install the plugin only on that subsite and not on the full network.
Please note that in this funky case you might not be able to get updates automatically. You will still be eligible, but you will likely need to install the updates manually.
Then on the subsite you can go to Events > Settings > Licenses and enter your license key.
Once you entered your license key wait until it validates and hit the Save button only afterwards. Otherwise the license key will not be saved.
Let me know if you have further questions. I’d be happy to help.
Cheers,
Andras