[Prestashop 1.6.18 >> 8.1] Upgrading Version But Missing Stock Features

[Aquavilla]

Where is my advanced stock system?

I can't seem to put find any relevant information regarding this anywhere. Our team currently uses the PrestaShop 1.6.1.18 version. I'm in the process of updating everything to version 8.1.6, but I've run into some major differences. Which is expected since this was a major version update. 

I don't necessarily need the advanced stock system but would like to know how someone goes about adding supply orders. 

https://docs.prestashop-project.org/1-6-documentation/english-documentation/user-guide/advanced-stock-management/presenting-the-stock-management-interface#presentingthestockmanagementinterface-supplyorders

How does one go about adding new stock with a different cost and unit prices for a bulk amount of products that already exists? I tried with the import feature, but can't seem to add stock to the existing product while taking in consideration the new cost for the new stock.

If this feature was taken out, would anyone have any recommendation as to where I can get a module to replace this functionality?