Allison: If I may jump in here with a "workaround" I use because I only have the Surface Modeling license.
Workflow:
Export the surface as an XML. as Erik says.
Create an empty project and drag the XML in. Turn off the surface. That leaves the linework. Export the linework as a DXF. New project and import the DXF. This will come in as 3D Polylines.
The next step sounds confusing but is pretty easy. It will convert all 3D polylines to linestrings at once.
Somewhere on the screen away from the tin lines create a text something like "100.000"
Click on Elevate Line (Data Prep Tab, Elevate, Lines).
Select the top option "Text Insertion". In the rest of that command window set the offset tolerance to .001 and the base elevation to 0.
Select your Text (ie "100.000")
Select ALL your tin polylines in the Lines box.
Hit apply.
All polylines become linestrings.
Export happily to SCS900.
Hope this helps/works.
Marshall