Hi Tomas,
Con(isnull(treeraster),(dem),(dem)+treeraster) is the one to use. The treeraster in this example is created from polygons that have an attribute of height (such as 15 m) and have been converted to raster using that attribute for the value field. This would result in a raster that has pixels with a value of 15 underneath the polygon, but NoData where you have no polygons.
Eric