Mosaic operations are for merging adjacent rasters. What you want to do is quite simple: 1) set your dem as the mask and extent raster; 2) add a field with values representing your obstruction height offsets; 3) convert your polygons to a raster; 4) convert the background nodata values to 0; 5) sum the two rasters.

Mosaic operations are for merging adjacent rasters. What you want to do is quite simple:

1. set your dem as the mask and extent raster;
2. add a field with values representing your obstruction height offsets;
3. convert your polygons to a raster;
4. convert the background nodata values to 0;
5. sum the two rasters.