1-Enable "Serial number" in the product form

2-Go to Inventory > Operations > Inventory Adjustments and start a new inventory.

https://www.odoo.com/documentation/user/12.0/inventory/management/adjustment/initial_inventory.html

3-For each product, you need to replicate the same number of line as the original quantity and set the real quantity to 1 for each line   and add (or create) the serial number

For exemple, if the theoritical quantity is 7, you create 7 new line for that product and location with real quantity=1.

4-Set the original line real quantity to zero (because you will double quantity)

5-Validate the inventory

If you want to do this through Excel, you could follow the same method, but after you have created and start your inventory (inventory in progress), you can go back to "Inventory adjustments" view, select the checkbox beside your current inventory and click Action > Export.

You can apply the same rules (point 3 and 4)

After, you need to Import the new inventory, Start it, and validate it...

A third way, could be also to use "Fake" internal transfert operation.

You need to make sure that "Create New Lots/Serial Numbers" and "Show Detailed Operations" are checked in Inventory >Configuration > Operations Types

After that you can create an "Immediate Transfer" (Internal transfers)

You have to use the same "From" and "To" location if you dont want to move your product.

You set (or create) the serial number and set the quantity "Done" to 1

You repeat the same settings by adding new line for each product and you validate...