Trading Partner - Affiliates not in our SAP Instance

Hello FI gurus,

I have been asked to set up Trading Partner Configuration on our ECC 6 system(New GL not implemented). We are the US affiliate of a Japanese company, and our Brazil and Mexico Affiliates are also in our same SAP instance as separate company codes. There are 4 other affiliates(including Japan) of ours who are not part of our SAP system(instance) and have their own ERP systems(SAP and non-SAP). In other words, these 4 do NOT have company codes in our system. All of us affiliates buy from and sell to each other. My question is: Is it possible to set up the 4 affiliates not in our system as Trading Partners as well OR am I restricted to just the 3 company codes in our system?

I understand that I have to set up Companies(Trading Partners) in Tcode OX15 and then as the next step connect these companies to company codes in TCode OX16 as the subsequent step. That is where my confusion came in, since I don't have company codes for those 4 affiliates outside our system. The user who wants me to set this up was fairly confident that it is still possible to set them up as trading partners, since that was how it was at her previous job.

Also, if someone can tell me what the true purpose of Trading Partner concept is, that would be great. Is this purely for reporting purposes?

Thanks and regards,

Wayfarer