Link topics after they have already been closed

Pretty much what in the subject.

As a user I would like to have the ability to link related issues. Items dealing with the same subject, or one has answered the other. There is this kind of feature on Jira, GitHub, StackOverflow among others.
The user could link the issues and submit to a community moderator to review and approve.

When the topic still open it’s possible to reply, adding the link to another issue. But when the topic is closed, it’s not more possible to do it. In this case, a topic keeps without a solutions even if it was resolved on another one.

Not exactly the answer you are after, but keep in mind links in Discourse are bi-directional.

As long as you link the destination to the source, the source will have a link to the destination.

So you could use an open topic to link up the closed topic.