To be able to add Topics and Collections as Sources to a Collection, you must first have a Collection created.
If you want to have articles from other Topics or Collections within your account to feed into another Collection, this is how you would do it.
Similar to adding an RSS feed as a source, you would navigate to the Sources section of the Collection creation (or edit your Collection) screen, and click Manage. Here you can also apply Filter Views to your sourced Topic or Collection. Want to learn more about Filter Views? Click here.
Once there, you would make sure to select Topics or Collections, and then select which Topic or Collection you would like to feed into your Collection!