You can now add owns links and texts to your navigation list (left box).
To activate this feature goto Login -> Manage preferences and set 'Extended settings for the menuentries?' to 'Yes'
Now goto Login -> Manage pages and create/edit a page. You'll see new options on the top of the page.
You have to create a new page for each entry.
For an external link create a new main or subpage (depends where the link should be in your navigation list). Choose 'External link' from the 'Type of this menuentry' list and enter the URL at 'Link to which address?'.