Extending the default dialog is an option, however it might involve more development on your end, as it's been custom tailored to the available options out of the box. What I'd recommend in this case is to develop a custom Link dialog for the ContentEditor and implement logic for linking to any content item you'd like. We have a resource showing exactly how this task can be achieved - please feel free to check Nikola's blog post on the topic
, hope you find it useful.
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