Update stock, prices, customers, etc directly into database

I am working with the developer who made “ERP” that our company has been using for the last 20 years. Employees still want the old program, but want to manage stock from erpnext and to proceed with furder onboarding from there. The developer wants to write stock, user and price changes directly into erpnext database.

I have glanced over db tables, and there seems to be quite a lot going on in every table. For example, if I wanted to update stock. One way I see this as doable is if I create a new row in Stock Ledger Entry. But I wonder If this will break something. Another, the most simple way I can imagine is to import CSV every hour or so. But this feels even dirtier than the previous solution.

The reason I don’t use API is that the old ERP development stack is unknown to me. I think it is made in Foxit pro. Also developer of that system requested to do it by DB insert.