Not sure what these others guys are referencing, HEC-RAS does the surface intersection and delineation between sections natively through RAS Mapper if you have a terrain in your model. I would trust the RAS generated results more than results interpolated by third party software (e.g. inroads)...
In the latest version of RAS (5.0) you can save the inundation limits polygon to a shapefile or KML, easily imported into CAD. This is done within the RAS Mapper portion of the program, under results in the study tree, where right clicking is your friend.