Hi Martin,
You can add custom properties to the workbook as shown in the below codesnippets:
ICustomDocumentProperties customProperites = workbook.CustomDocumentProperties;
customProperites[ "Author" ].Text = "Essential XlsIO";
customProperites[ "Comments" ].Text = "XlsIO support Custom document properties";
You can refer to the below documentation link that illustrates the same:
http://www2.syncfusion.com/ug_64/xlsio/DocumentProperties.htmlPlease let me know if I have not understood the requirement clearly.
Regards,
Geetha