I imagine doing this is inadvisable, as your changes will get overwritten next time you update your theme, but adding extra Glyphicons is possible.
You need to edit /wp-content/themes/enlightenment/core/functions/menu-icons.php and add new lines in the $icons array for each extra Glyphicon you want.
For example, to add the ‘Save’ icon I added this line at the end of the array, after the ‘time’ line:
'save' => __( 'Save', 'enlightenment' ),
The full list of Glyphicons is here.
Ben