Yes it's simple.
As long as the sales order line is at Status Allocated (Receipt Note must be Completed from the Purchase Order) and the Status of the Despatch Note is NOT Despatched.
Within the existing Despatch Note there is button at the bottom left of the screen Available Items.
Click on this button and any available Despatch lines will be visible in the pop up.
Select the line(s) to add to the note and click Add Items.