Access CustomDocumentProperties from excel in c#

I have following code in xml:

xmlWriter.WriteRaw(@"<CustomDocumentProperties xmlns=""urn:schemas-microsoft-com:office:office"">
<MSCRMCreatePivot dt:dt=""float"">");
xmlWriter.WriteRaw(isExportPivot ? "1" : "0");
xmlWriter.WriteRaw(@"</MSCRMCreatePivot>
</CustomDocumentProperties>");

I want to access CustomDocumentProperties for excel which will function similar to that in xml. Please let me know how I can set the properties for excel using OpenXml.