Levelling an existing site in DSM for viewshed analysis in GrassGIS

I’m doing a multi-point viewshed analysis for an urban site in GrassGIS based on a LIDAR DSM raster maps, to test for a series of proposed buildings. The site has existing building on them.

Is there way of "levelling" the site to the surrounding ground level itself, so that the calculated height would be that of "proposed building above ground", not proposed building above existing buildings would be used?

In my ignorance I have tried to export the DSM to PNG, then then painted the site in in Photoshop, with the colour of the surrounding terrain, hoping that I could re-import it and use it as a basis for viewshed. On import, the PNG is broken into r. g. b . channels, not giving me one map.

I have access to Autocad,so I was hoping there would be a way of drawing up the site boundary as a vector, and using that as a boundary to redefine the level inside the site.
Or any other approaches inside of GrassGIS that achieve the same result.