Currently, the SdtProperties classes (ex: DateProperties) can be extended by overriding the GetBuilder() method. This allows you to implement custom ISdtBuilder that creates the document content of the SDT form. However, the custom properties are not taken into account during import/export. Consider adding some kind of custom information about the custom SdtProperties type during export and restore it during import. Or at the least provide API (like an event or method override) that can intercept the creation of the SdtProperties during import which will allow you to replace the default SdtProperties object.