You mean like raising it in the entire map, or just at one location? If it's just in one location it shouldn't be too much trouble just using the regular terraining tools to get it done. If it's for the entire map you should probably think about it before creating the map, since when you select the option "New Map" from the "File" list, you can decide on which default terrain level the map should have over the entire map. Of course, that way it gets done instantly, but I don't think it can be done after the map has been created.
EDIT: A method which might give sort of the result your looking for is to use terrain deformation actions in triggers. By setting a negative value in for example the "Create crater" action you can increase the level of the terrain, but I've never tried this myself and I'm not sure exactly how the result will be, probably just a bump and no cliffs, so it depends on what you want.