Use the Purchase Order section to transfer stock between warehouses.
Start by going to the Purchase Order section from the left menu. Create a new Purchase Order. Where you normally see the Vendor field, there is text below that to choose a warehouse. Click on 'From Warehouse'.
Then you can chose the origin warehouse. The next field will have you chose a destination warehouse. Add products as you normally would on a purchase order.
Once the stock arrives at the destination warehouse, you'll receive it just as you would a PO.