Adding Inventory quantities in Inventory Management.

We're on version 5.20.2.0. We have not been maintaining inventory of finished goods in the inventory module. Before we do a physical inventory and add the quantity, I wanted to see if this will effect any other part of the system or can we go ahead and add? I'd also like to get thoughts on adding our raw materials inventory so we can better track when we need to order as inventory for replensihment.......thanks