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.
Use the below script to delete the registed build process template in Database:
use Tfs_YourTeamCollectionName;
delete from tbl_BuildProcessTemplate where ProcessTemplateID = 'Your ProcessTemplateID';
Create new build definition again.
The database name is your TFS team collection name e.g. Tfs_YourTeamCollection in the backend SQL Server of your TFS server. I'm using TFS 2012.
Hope it helps!