Hi Richard,
Your Developer tab should have the following. If not, you need to install the add-on.
Once that is good, open GP, run the Report Writer report one time, and output it as xml. Save that file in the folder where you are putting you template. Once you done this, you need to load it into the template add-on with that "Add Source" button next to the "Field List." Point to the new xml file, and it will update the resource pull-down on the left with you source file. This will add the new fields you put into the modified report. Then, drag and drop them in.
If you do not remove the original source first, you will have two sets of the same data with one having the extra fields. You can remove the source, but I don't know if you will lose the link from the xml and your template forcing you to add all the fields to the template all over again. You probably don't want to do that. Save a copy of your existing template in case you break it.