The instructions for making the CSV file for s2member say
If you fill the ID field, the Import routine will update an account matching the ID you specify ( so long as the account ID does NOT belong to an Administrator/Editor/Author/Contributor, this is for security ). When importing new Users/Members, you can leave the ID field empty; don't remove it, just leave it empty ( i.e. "" ).
WishList's member export doesn't have an ID column, just username (WP username). I'm working on a site with 300 members (and they need their WP user accounts because the site is a protected Simple:Press forum), if I run the s2member import routine without IDs being matched to usernames will s2member match them up on its own?