Your Jira Server license allows you to update your instance and get support as per your license type considerations.
In general you can install plugins/apps (depending on your jira version they will be called differently) regardless of your jira licens status (I've done that in the past). Of course you'll have to be extra careful checking the compatibility of each plugin/app (you should be anyways so this is not a big deal).
One thing you have to consider is that many plugins/apps require a certain license tier, and most of the times this has to be equal or bigger than your jira license tier. Soooo... in short:
You SHOULD be able to but check for each case. If in any doubt contact the plugin/app developer directly.
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