Hi.
How would you like us to advise if we don't have any information about your POS system and support.
Each product in the e-shop has a unique identifier and that is the product ID.
Furthermore, it can be used as a unique reference identifier (can be set for combinations) or EAN 13 (can be set for combinations).
Can your POS system do it?
Export us an XML file with products from your POS system (two are enough, where one will be a combination).
Show us your XML that you import into Prestashop via VB6.
I really can't advise you that way.
You give us little information or no information and documents.
This is the same as if you write an email to a car repair shop and tell them that you have a car and the car is red and you have a remote control and a door in it and they won't open and close.