We were looking to do the exact same thing on the cloud instance that we have but see no way to tag other author's threads in a group we manage.
I am looking for a way to categorize the threads and it is not feasible to force users to tag threads using an appropriate tag for their issue from a list of tags we provide.
Categories in Jive do not help either as there too assigning a thread to a category cannot be forced. Also as a workaround even if I wanted to I cannot assign threads by others to categories as I cannot edit the threads even though I manage the group.
Looking for a solution on categorizing threads here.
I proposed a alternate solution in Provide structured Tagging to enhance data validity
Check it out and help flesh it out to cover your use case.
I guess I should be encouraged to know that I am not the only one that finds Jive's meta tagging of questionable value