In your server ...
http://yoursite/admin/settings.php?section=manageauths
There is a setting for
Allow log in via email authloginviaemai which has as a comment:
Allow users to use both username and email address (if unique) for site login.
However, if you are using the same plugin I am, it still might be possible to use a csv file from your student information system ... even if it doesn't use 'social_user_#' by manipulating the csv yourself.
Like I said the plugin I use gives those usernames sequentially ...
social_user_1
social_user_2
etc..
One could find the last social_user_# entered in the mdl_user table, and in the csv file you to use to import **new** students add a column in a spreadsheet app that concactates 'social_user_' with the next number sequentially beginning with the last number seen in the dump of the mdl_user table.
The problem stuff would be a student already enrolled in the moodle who had a name/info change in the SIS.
How 'flexible' is your SIS? Maybe a field could be added that matched the requirements for social_user_# where # is sequential.
Don't think there is an easy answer ... without some human manipulation. However, maybe if you study it a bit, you might come up with your own idea and run it by whoever runs your SIS.
'spirit of sharing', Ken