To add a form field:
1) update your database to add a column in your users table associated with the new field you are adding.
2) you could copy and paste to create new rows in the table, or simply use Dreamweaver to add new form fields. If you use Spry fields, they will have similar client validations.
3) If you want extra security that you can't get with client validation, then you can also update the server validations using the Server Behavior applied on the page (Assuming you have Form Builder installed).
4) Update the Insert Record Server Behavior that inserts the field to the database (if you have DataAssist installed)
5) Update the Insert, Update, and Profile pages to add fields similarly.