Thanks for all response!
MED is already capable of importing colored hightmaps, and I experimented with this feature. This gives me more than 16 million hight values instead of 256 and I could make of course very fine slopes, without the stairs I mentioned above.
But is there a terraineditor, that exports such heightmaps? This would be the perfect solution.
I use earthsculptor but it exports only grey values, which is the bottleneck. Does anybody know of a workflow with colored heightmaps? I mean a terrain editor, that can export them? Is there a common standard for the weighting of the colors?

I hope, this doesn´t sound too complicated, I´m actually searching for a good workflow.
For the moment it might be - as Jcl suggested - the best method to make models and convert them to terrain. But nevertheless I would like the implementation of a "smoothing tool" in MED ...