If you want to use SharePoint links besides documents, you have to enable Content Types for the Document Library. This is done in the Advanced Settings menu. In the Content Types section, select Yes to allow multiple content types.
In the section Content Types in the general settings, you may now add the content type Link to a document in a similar manner. The columns then have to be configured for documents and links separately.
Finally, you have to configure both Content Types in conf/config.xml by the properties document.content.type.name and link.content.type.name.