AbleCommerce Shopping Cart Software

Is it possible to export some user-defined data that is associated with a user (e.g. a "VATNumber")?

I have created an XML template for the download based on the default template - but how do you add a mapping for "Settings > UserSetting > {fieldname}VATNumber"?

Those templates don't allow customer to add extra fields. Mapping here means that you can map available AbleCommerce fields in right side to the left ones. You can not extend these AbleCommerce fields to include more fields.