[tubedogg]

You can definitely add fields to the user table; however make sure you give them descriptive names so there's less chance of duplicating a future vB column name.

When deleting a user, you should run the following queries:
DELETE * FROM user WHERE userid='XX';
DELETE * FROM privatemessage WHERE touserid='XX';
DELETE * FROM access WHERE userid='XX';
DELETE * FROM customavatar WHERE userid='XX';
DELETE * FROM calendar WHERE userid='XX' and public='0';
DELETE * FROM moderator WHERE userid='XX';
DELETE * FROM userfield WHERE userid='XX';
DELETE * FROM subscribeforum WHERE userid='XX';
DELETE * FROM subscribethread WHERE userid='XX';

If you want to delete all posts by the user:
DELETE * FROM post WHERE userid='XX';
DELETE * FROM attachment WHERE userid='XX';

If you want to delete all poll votes by the user:
DELETE * FROM pollvote WHERE userid='XX';

If you want to delete all threads started by the user:
DELETE * FROM thread WHERE postuserid='XX';