For a build process template, we can add/remove/edit the argument list of it and use it as variables within the build/work-flow steps. I reading the nice guide
Within the arugments there is a property called "Metadata" as shown in the picture below. Click on the button at the right end and a window should pop up as shown in the second picture. Enter the name of the new variable in the Parameter Name and enter the other details (Display name etc). Save and check-in the build definition and you should be good to go.

