Hello Gerhard,
Thank you for posting your question in Bentley Forum.
I presumed you are using Open Roads Technology, you can use Export to Native under General Geometry. Yes you can select your feature separately and export to MXRoad.
If you want to export your triangulation model then hoover on it and you can export it to MXRoad.
Please let me know if you have any questions on the same.
Regards,
Purva Lakde