Matching customer sku to our sku and putting it on sales order

We want to take our customers' skus and match them to our skus. Basically, cross-reference our customer's skus to our skus, then put both on the Sales Order to make it easier on our customers.

My thought was to create a custom record with our Item and Customer as Parents, then have the Customer's Sku. This makes it easy to create a cross-reference.

My problem is, how to get their sku on the Advanced Sales Order PDF in a column next to our item's sku? I'm quite confident in editing the Advanced PDFs. I was thinking I could add their cross-referenced sku as a line-item value on the Sales Order then source that from the PDF. I've done something similar in the past, but I've never had to match two fields, both the customer and our item from the cross-reference table, to get the correct customer sku.