Add closed tickets to a problem
Closed tickets are the first source to identify a problem.
Unfortunately GLPI don't authorize to add closed tickets to problems.
It's a pity, enable this feature would be useful to document problems with old closed tickets.
Customer support service by UserEcho
I used differently the system : I never close tickets so I can add them to problems. To do this, you can fidget with rights in the ticket lifecycle, and uncheck "Closed" in the last column of Home / Administration / Profiles.