How do I add a N2 device to a S4 Open Appliance?

You have two options:
1) Run the configure wizard again, or
2) Manually add the device

If you manually add the missing devices you have to do 4 steps for each one
1) Add the device to the correct physical network

2) Add the device to the Building Control Network (BCN)

3) On the Physical Device tab in the BCN object link the two together

4) Now, manually select the correct Device Template to the object in the BCN