RE: Add new fields and methods in standard table in D3FO

In your own package/model create your own extension of sales Table and add your fields. Changing any standard objet is not allowed from 8.0 onwards. For adding methods you have to create extension class for the table , you can not add any methods on extensions.

See below syntax

[ExtensionOf(tableStr(SalesTable))]
final class XYZSalesTable_extension
{
    //Your Method
}