Combining external (webshop) invoices with your own sales invoices

Can you combine external (webshop) invoices with your own sales invoices? Our webshop has its own numbering system, which we set high enough so that we never had overlapping numbers. In the webshop, there was an invoice number (internal, we couldn’t edit it) and an order number, which we then created an invoice number from. How do you achieve consistent numbering here?