You can do that from -> Warehouse > Inventory Control > Inventory Adjustments

Create all the products for which you want to keep the stock (as stockable products). Once the required products in stock have been defined, use an initial inventory operation to put the current quantities into the system by location. Go to the menu Warehouse > Inventory Control > Inventory Adjustments to enter your initial inventory.

Give it a name (for example Initial Inventory) and select the stock location for your inventory. Note that when you select a parent location (such as Stock, which might be split into sublocations), you can also select the sub (or child) locations. You can choose between making an initial inventory for All products in that location, or for one product only. In this case, we choose the All products option. If you need your stock valuation to be done in a different period than the one that will be selected by default according to the inventory end date, enter the corresponding accounting period in the Force Valuation Period field.

In case of the initial inventory that is drawn up when you start in Odoo, no stock is available yet. Click the Start Inventory button and add each product by clicking the Add an item link. Select the corresponding stock location if it differs from the default one. Add the Real Quantity that you have in your stock for each product.

Click the Validate Inventory button to confirm the initial inventory and post it.

On the Inventory Adjustments tab, notice that the Source Location is automatically set to the virtual Inventory Loss location. Odoo always needs a source location and a destination location.

Refer this link : Initial inventory

Hope this helps...