There is another workaround possible.

Let's assume your counter top is 4 centimeters thick.
- Using very thin walls with 4 cms height, trace the outline of the countertop.
- Create a room inside the walls
- Export, then import your kitchen sink, and make sure to check the Staircase checkbox during importation.
- When you insert the sink, it will cut out a hole in the ceiling/floor.
- Create a new level and adjust elevations so that things line up properly, and the sink is flush with the counter top.
- Export the whole thing and import it as a normal piece of furniture.
- Place it on top of (a) kitchen cabinet(s) without a counter top.

It's a bit complicated to do it this way, and it may be easier to use existing models.

Hans