I followed the instructions given by whuber (How to build an artificial 'dam' across a river? and Add a dam to DEM in ArcMap) and ended up with this model:
With this model I'm able to modify my DEM to generate a dam which has a profile like this (Settings: Elevation = 5, Slope Ratio = 1/1):
Now I need to add a "crown" (thats the literal translation from German) with a defined width to the dam. It should look something like this:
How do I have to modify my model to achieve this?