Hi,
I looked into this, and it seems like this is just something that has not been developed yet. It is definitely on our to-do list though (task #258727).
Unfortunately I can't think of any way to export a 2D representation of the stair tot IFC. If you find a way, I would very much like to hear about it.
Kind Regards,
Daniel Alexander Kovacs
Professional Services Consultant
GRAPHISOFT
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