You can only add one external editor manually.
If it is a third party plugin which is supported in Lightroom 5, it would be automatically recognized as an external editor by Lightroom once you install it.
You can add as many as you want. It isn't intuitive, but that's the Lightroom way, sometimes. Start by hitting the Choose button:
That takes you to a dialogue where you navigate to the .EXE file that you want to use as an external editor. Navigate there and push the Choose button.
I chose FSViewer.exe, and when I came back, I saw this:
Note that the application has changed, but the Preset has not. Now click in the Preset box and see this:
Click on the Save Current Settings as New Preset... line. In the box that comes up, give a name and hit Create. Congratulations, you've added another external editor. If you have more, repeat the process.