BPMN is not supported for import to Business Studio. The export is designed to take the simple notation for Nimbus and import to Business Studio to then elaborate further into BPMN / executable format. Note that the limitation here is on the Business Studio import side, not the Nimbus export side. The import utility that was built was only done to use the standard / simple / Universal process notation format and not BPMN format from Nimbus. Nimbus uses the same simplified XML export for both notations but Business Studio import does not recognize extra elements from the BPMN format. All ensure that the option "Simplified XML export" is selected when exporting to XML.