RSECTION cross-sections can be defined using a model template in Grasshopper.
Create template with RSECTION cross-section
Import your RSECTION cross-section into RFEM 6:
Then save the RFEM file as a template (*.ft6 file):
Refer to the template in Grasshopper
In Grasshopper, you can access this template file by storing the path of the saved RFEM template file in the export component (Use Template).