Yes, as long as you have 'space' on the Agent License, you just need to click on Add Agent.
For more details, take a look at the documentation at https://confluence.atlassian.com/display/SERVICEDESK/Managing+agents
UPDATE:
To see a customer on the Agent screen, you first need to edit the user and make sure he's in the JIRA Users Global Permission.
-- Pedro
This appears to not work. The add agent screen won't find any user that is a service desk customer.
My bad, before you do that, you need to ensure that the Customer belongs to the groups that can access JIRA
Still no dice. Any user marked in admin / user management as a "SD" user is not able to be found at all. I am unable to add those users to any group in Jira nor can I grant them Jira application access.
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