You can git fetch
any repository URL into any other repository. By default this will not leave permanent traces, i.e. it will download the objects but won't alter branches or other refs (as long as you don't get 'fetch' and 'pull' mixed up).
git fetch https://github.com/foo/bar
Note that this is done without adding a remote. (Although if you wanted to have branches from both repositories, then you could add a remote for the other repo.)
This works best if the two repositories have related histories, but that's not a requirement – fetching an unrelated repo will need to download all its data but won't otherwise harm the local repository. (If you fetch a URL without creating a remote, then all fetched data will get garbage-collected sometime later, as no branches reference it.)