Is it possible to implement a field on a page which relates to both pages and content from all my collections?

I can only find the fields "page" or "collections". I want to be able to point to any content on my site that has a URL, from one field in my dashboard.

The purpose is to be able to populate a "sub menu" per page - so if this is not possible and there are any other suggestions for how to implement this, please tell.