relationship for importing lines from Bento

I'd be grateful if someone could kindly help me... I've created a database with tables for "Customers" "Invoices" and "Lines". I have relationships as follows:

customer::kp_customer_id = invoice::kf_customer_id

and

invoice::kp_invoice_id = line::kf_invoice_id

On the Customers layout, I have a portal showing related records from Lines - and this seems to work fine and lists all the lines for each customer.

I have also imported the lines from my old Appleworks database via Bento. The old database was not sophisticated and had the customer details on every new record. I have imported the customer details and line details separately to the appropriate tables.

However, I'm really struggling to find a way to get the old lines to show up in the portal on the Customers layout. I thought I would be able to do it by adding the customer_id to each line but I don't know what relationship would make this work.

Any help would be most gratefully received.

Thanks in advance,

Michael