You can modify what is already there. Best thing to do, however, is to copy the default receipt template to a new file and modify that copy of the file. Then, on the Preferences tab, select the Receipts section and change the receipts template to point to your copy. That way the software is using your copy of the receipts template instead of the original file that comes with the software. And since it's a copy of the original file you don't have to "start from scratch". The reason you want to do this is if you change a file that is included with the software, your changes could be overwritten later if/when you upgrade the program to a newer version. Any other files you create won't ever be touched by the software during an upgrade.
If you look in the user manual included with the software, in the "Exporting Data" section (section 35 in my manual, which is for version 3.3), you'll find a section for receipts. You can include items like <firstname> and <lastname> in the receipt to include the participant's first and last name. There are a bunch of other items you can include as well.