Use SetCustomProperty to create a custom document property. Pass the method the name and value of the property. The value can be one of the following types, depending on the property: String, Double, Integer, Boolean, or DateTime.

 public void SetCustomProperty(System.String key, System.Object value)
Public Sub SetCustomProperty(ByVal key As String, ByVal value As Object)

The key or name of the custom property.

The value of the custom property.

docProps.SetCustomProperty("Reviewed by", "J. Smith");
docProps.SetCustomProperty("Reviewed by", "J. Smith")