As per https://eduforge.org/tracker/?func=detail&atid=739&aid=3388&group_id=176 (Mahara bug 3388 - Student ID is not passed from Moodle (via XMLRPC))
When roaming from Moodle 1.9.5+ (20090804) to Mahara 1.1.7testing (2009022620) (via XMLRPC) the student ID is not passed through.
I think maybe the reason this isn't passed from Moodle, is that
this is a potentially conflictey field.
Local users may well have their own naming scheme that is the
way that institution handles idnumbers and they might not want
remote users coming in with this field, where their idnumbers
may conflict with the local naming scheme.
Maybe we should file a bug about it on the Moodle tracker and
discuss there having options to send it or not.
So here is the bug . I'm afraid I don't know much about the student ID field, so I leave it to you Moodlers to decide whether it can be passed