To add a new hole to an existing layout, open the Map Manager tool and tap on the Actions button (••• on mobile) in the upper-right. Select + Add New Hole and you'll be prompted to input where in the hole order this new hole comes up. For example, if it's a new hole 19 that tacks onto the end of the normally 18-hole course, then you'd input "Hole 18" for the Hole Comes After field.

Next, you can tap on the + Add New Tee Position and + Add New Target Position buttons to put the new hole on the map, and select which hole they apply to (Hole 19, in the example from the above paragraph). Make sure to label everything!