@Natalie1991 welcome to the KNIME forum. I once built a more complex header with the help of OpenPyxl and KNIME
There are other tools out there there that can help you create the Excel format you might want. You can read more about what you can do in this article:
Another option it if is to stay within KNIME or you want some interactive elements is to use DataApps (Components) and maybe later export them as a Report / PDF (and send them).