There is a tool to create 3D linear features from a plan element with profile elements belonging to it. I don't know that you have the same button setup as I have, so I'm not sure where you'll find it, but I think it's called Create Additional 3D Element or something like that. From there, a 3D element will be created within your 3D model.
If you will only need one profile, you'll be able to get away with skipping the previous step I mentioned. You can instead select the profile element and assign it as the Active Profile. From there, the 3D element will be created and will follow any change you may make to that profile or plan element.