You can certainly open the Append Report Text vi and modify it as you please. You should rename it though. If you ever re-install the report generation toolkit, it would overwrite your changes if you didn't rename it. However, I don't know how you would go about centering the lines you want to append. Perhaps counting the number of characters in the line to determine the number of blank spaces needed to be inserted in order to center the line.
Counting the number of characters and the space only works if you are using a monospace font. The default is the application font, which is a porportional space font. That process generally does not work too well.