[Imp]

Set the fill range to a high level, select the terrain you want in this case level 1 hill & press the fill button. Do a few times across the map till its all at that level.
With hills the map edges may still show slopes in which case save it & reload it & that will get rid of them giving you a flat map.

Turns streams into deep cuttings & makes LOS into rivers much harder, you can spend a bit of time making some gentler sloping terrain into it in places using the asterik key & or marshes. i.e. set some hexes to 2 4 6 8 for a gentle slope.

I ocasionaly do it on the odd hill esp if a road is going up it or for raised, sunken road & raillines.

No you cant do it managed it once somehow using the asterik key & a multi hex building.
The trouble with raising the height of the building is you produce a hill & hence slopes so for urban enviroment ruins LOS for any adjacent road unless thats what you want.

Save your map before messing with the asterik key it can sometimes cause problems forming a ripple & producing invisible slopes elsewhere.