Adding Wemo units used to be much more difficult. They have certainly improved the process.
There are a few things to do to add a wemo to overwatch.
First is to setup the wemo using the wemo app on your phone. This allows you to retrieve the IP Address of the unit.
For this example, I will be setting up one of the smaller wemo wifi smart plugs.
Configure new wemo
Open the wemo app on your phone.
Click on the
gear icon at the top.
The first option is
"Add a wemo", click this.

This will take you through all of the steps to add your wemo to the list.
Once this has completed, your new wemo should be in the list, if not repeat the process.
Click the on/off to test that it works.
Getting the wemo IP
Click on the
gear at the top again.
Click
Settings & About in the list of options.
Click
Hardware info
Select
the device you have just added, it should be in the list.
Scroll down to IP Address and copy the IP address down.
Adding to Overwatch
This will be specific to your overwatch server setup.
ssh to your overwatch server.
Go to the overwatch web directory (/usr/local//wemocontrol)
sudo nano wemobackend.py
Edit the section where the wemo list is and add your new wemo name and IP:
wemos = [
("192.168.1.10","wemo1","Wemo1"),
("192.168.1.11","wemo2","Wemo2") (your new wemo IP, shortname,display name)
]
Run the python script so that it rebuilds the wemo database
python wemobackend.py
If there are no issues, it will respond back with the status of each wemo in your list.
Log into overwatch and your new pi should now appear in the list of wemos.
Configure Router
To insure that your wemo retains the same IP address each time it is rebooted, you will want to update your router to reserve that IP address in DHCP.
This is normally done by logging into your router and under the dhcp settings, edit the dhcp scopes.
You should be able to see a list of MAC (machine addresses) and their matching IP addresses which have been given out.
Add the MAC address of the wemo to the reserved IP list. This will insure that each time your wemo boots, it is given out the same IP and it will not offer that IP to any other devices.
You are done... enjoy