Use the PressurePipe.Curve node from the Civil 3D Toolkit to get the curves (lines and arcs) for the pipes. Then you could either add those to model space as feature lines and then explode them (which will create tessellated 3D polylines) or just tessellate them first in Dynamo and create 3D polylines directly using Polyline3D.ByPoints. It doesn’t matter when you do the tessellation, but it will have to happen at some point in the process because InfraWorks can’t handle true curved pipes anyway.
You could look at my code here to try and automate the MAPEXPORT process.