Hi Piersante!
Another Moodler [Rudy Scott] a while back attempted to create a "Subsection" module. But I do not think it was ever fully developed. You can check it out at the following links. Remember he was working with older Moodle code. Maybe v1.3 or v1.4.
subsection starting point
http://moodle.org/mod/forum/discuss.php?d=5379
return of the subsection: testing wanted
http://moodle.org/mod/forum/discuss.php?d=10357
WP1