How can we auto generate the Customer Number, Site Number & A/c Number by using Customer Spreadsheet

How can we auto generate the Customer Number, Site Number & A/c Number by using Customer Spreadsheet.

Customer FBDI has the option to not to populate Customer Number, Site Number & A/c Number (Optional Fields). But Customer Spreadsheet has these field compulsory. Business user wants these numbers to be generated by system automatically but unable to do so because these fields are compulsory in Spreadsheet.