Thanks for reporting this important issue.
@ Erwan Maizy
s2Member does support this, however it appears this functionality is broken in WordPress v3.5, so we’re having this corrected now for the next maintenance release. Thanks for your reporting this!
If you don’t want to wait for the next maintenance release, please unzip and upload the attached file, allowing it to override your existing copy of: /s2member-pro/includes/classes/imports-in.inc.php
PATCH file: http://d1v41qemfjie0l.cloudfront.net/s2member/uploads/imports-in.inc_.php_.zip
Additional Details…
It’s possible to import new Users into a Child Blog where they do NOT exist yet (i.e. they’re not part of that Child Blog yet). In order to accomplish this, you MUST leave the “ID” column empty, telling s2Member to create a new User, and not update an existing User. I know this is confusing on a Network, because the User does exist, but they may NOT exist on a given Child Blog yet, so you will need to treat it as such (e.g. a new User).
How this is handled by s2Member’s importation routine…
If the User’s email address/username already exists on the Network elsewhere, s2Member will automatically detect this, and simply ADD the existing User to the Child Blog you’re working from (i.e. the Child Blog where you’re doing the importation). Just be sure to leave the “ID” column empty for this User in your import file.
"","johndoe","passsword","John","Doe","John Doe","johndoe@example.com"
Please let us know if problems persist :-)