Does a local GitLab instance allow forks from other repository providers?

I have a GitLab installation in our intranet, which is currently not visible to the Internet via HTTP / HTTPS.

GitLab offers to import existing repositories from GitLab.com, GitHub.com, … This feature is only available if I share a token with the partnering Git provider and offer callback URLs. (The internal GitLab is not accessable from Internet, thus this method can not be used yet.)

If I would enable such an access, would my internal GitLab instance keep track of branches from the forked repository like between forks on GitLab.com or GitHub.com itself? Would be possible to create merge requests?

Note: This is not a migration.

This question is not covered by documentation: https://docs.gitlab.com/ee/user/project/settings/import_export.html