Yes! You can add a fork as a standalone board or as a connected repo on a multi-repo board.
If you want issues from your forked repo to appear on your primary repo board, you'll need to add that forked repo as a connected repo to create a multi-repo board. You'll go to the sidebar for Project Settings->Repositories and click the green GitHub icon to add that repo. For the forked repo you're adding, you'll need to be a collaborator on that repo in order for issues to show up.