You want to modify the Seller Registration form (built-in form) on the site and add new fields that your customers can fill the information which you seek.
StoreHippo allows you to edit an existing form. If you wish to add a new field, you will need to save the data of the new field. For this, create a metafield which will be used to store the data of the field created. Follow the
Please follow these steps to edit a form:
Click on the following link to know more about forms: https://help.storehippo.com/topic/forms