4
You should be able to link to any "outside page" within an XOOPS navigation block through
http://yoursite.tld/xoops/modules/system/admin.php?fct=blocksadminIf you want to include content inside the central XOOPS blocks, you may want to create a new module, with content selectors.
content module -> selector ? -> selection ? content
For example, you could write an HTTP Error module that would encapsulate the error messages within the XOOPS framework. The content would be different, depending upon the HTTP Error code as the selector.