Hello,
Okay ; you want to change the order number format !
Because you were talking about add information to the invoices I thought that you wanted to customized the invoices.
So, yes in HikaShop Essential you can change the format for the "order number" and "invoice number".
And you can use a special tag to access to user data, as you wrote:
Now if you want to use something from a user custom field, you need to replace "user_vat" but the namekey of your custom field.
The namekey is also called "column name" :
www.hikashop.com/support/documentation/1...ield-form.html#infos
Afterwards, please note that when you change the format, it will not affect the old orders and you need to create new orders to see the result.
Regards,