to make these sorts of modifications to the registration page, you will need to use the following WebAssist Extensions:
CSS Form Builder
after you have those two extensions installed, you will be able to edit the Server Validations server behavior and the Insert Record server behavior.
Go to Window -> Server behaviors. In the list of server behaviors, double click the server validation server behavior. Here you can remove validations for form element you wish ot delete or add validations for form elements you have added.
Also in the server behavior list is the Insert Record behavior, double click this to remove user table column bindings to form elements you may delete or add new bindings to form elements you add.