I am having the same problem having a sunken sink filled in by the worktop.
I hope there is a better solution that someone can share with us? (I tried importing sink as a window or stairs to no avail), however, as a workaround I created a wall the same size at my worktop, I then inserted a service hatch with an invisible frame the same size as my sunken sink. I exported it as a .obj file. I then imported this as furniture and rotated it to horizontal and then fitted the sink.