have you looked into Boost navigation fumbling?
"Custom nodes can be nested with one hierarchy level, i.e. a custom node can have children nodes. The create a child node instead of a parent node, just prefix the custom node title with a hyphen.
For example:
Administration|/admin/index.php
-List of Moodle users|/admin/user.php
-Manage courses|/course/management.php "
looks like this:
