Hi denisp ,
Thanks for your question.
I'll assume that when you mention ATUM Inventory, you are actually referring to ATUM Multi Inventory to respond to you. Please let me know if I'm wrong.
ATUM Export Pro stores the old product ID in a custom meta when importing products using the "INSERT" mode. Then, when you import the inventories, the old ID previously saved will be used to relate the new inventories to their corresponding products.
Please note that this process can only be executed once. You won't be able to synchronise the products and inventories info after the first import process.
Best Regards,
José Andrés