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 ametafield which will be used to store the data of the field created. Follow thebelow-mentionedsteps to add a new field to your existing form: