Linking content type nodes with each other
You can actually "link" nodes at any time, you just have to edit the node to add the reference.  

If you want to add multiple references, you have to specify this when you set up the node reference field in your content type.  In the configuration page of the node reference field, choose how many references you want to allow (1-10, or unlimited).

For a user to add a reference that did not create the node, make sure the correct permissions are set in admin/user/permissions for the given user role. (ie "edit any EXAMPLE content")