To STOCK IN items/Products in bulk all you have to do is follow the steps mentioned
Step 1 : Click on inventory >> Bulk Item Stock In ( this option is avaliable on the top right corner)
Step 2 : Select the items you wish to stock in >> Next >>
Step 3: Enter the Quantity you want to stock in >> Submit
Step 1 is displayed above 
Step 2 is displayed above
Step 3 is displayed above