When doing Jira import with a shared user base on LDAP, it makes sense to use the email address as pivot as it's, most of the time, unique.
At Jira Import (issue type or project) and if LDAP is being used on Tuleap. If user email doesn't exist on Tuleap, we will look at LDAP. If value is unique at LDAP side, the user is automatically provisioned in Tuleap (as it's done when people not already registered as being added as project members).