What is the common party id for the same customer & supplier, but created separately

We have old suppliers & customers, which were created separately. Ideally, it should have been created as supplier as customer (12.2.5).

Our challenge is, we are in the process of developing one daily interface to push supplier & customer invoices from third party system. In face, we will have to process AP/AR netting for those parties & need to report the balances back to the third party system. So we require a common identifier since party is the same. However, those are not created supplier as customer.

Kindly assist.