ok
1) User-ID can't be empty, but if you'll take the NULL operator, it will use the autoincrement for that, so it works.
2) yes, you have to fill the userfield table too, or you cannot change userinfos in the admin-panel.
3) you can create the datelines with the php-function mktime(), look at php.net about the params
4) if the users are in the DB, you don't need to do anything more in the ACP, they are in