Multiple legal number sequences

In our Spanish company we use Legal Numbers on invoices/credits which works fine. It is set to automatically generate a number on posting the invoice. But we now have a problem that for exporting we need to not always generate an invoice number or, ideally, have a different sequence by customer. Obviously we can switch to manual generation but because of the quantity of orders we have, we prefer to have it automated. Is there any way this can be achieved?