To add a remote community to your forum index, you'll want to go to ACP > Manage > Categories

Under the "Add Category" dropdown, you can select the option to add a remote category.

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Then you can treat them like any other category (re-order, rename, etc.)

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The page you're on now, the category synchronization, allows a category to follow another category, and automatically cross-post its posts to that category. It also kind of does what you want, but it'll be more straightforward to add the remote community directly.