Hello Ron,
Thank you for your post.
Yes, in order to add a new field to the invoice you should modify the appropriate .sql file.
Please find the attached .sql file, where we added a New Field - 'gift_message'.
After the sql file has been modified, you have to paste newly added filed into the .fr3 file to make it displayed at the invoice.
Also you're welcome to find a detailed documentation for Fast Report Tool here:
http://www.fast-report.com/en/download/documentation/
Feel free to contact us with additional questions.