Or another possibility using surface offsetting similar to described before could be to select these 3 faces:



Then run Construct > Shell and choose a pretty small thickness value. That will leave those faces in place to become openings, and the surrounding surfaces will get offset and intersected with the original surfaces, making edges like this:


So then select those edges, use Ctrl+C to copy them to the clipboard, undo to revert back to the pre-shell state, and then use Ctrl+V to paste in the curves you want.

- Michael