Nope. Since Milkshape works way different than Hammer, you shouldn't load map files made in Milkshape. Hammer works with brushes, Milkshape works with faces. I'll go do some testing on this but I think it's a bad idea to exchange the two.
There are some terrain generators that create terrain suited for Hammer. Use these instead. Nem's terrain generator:
http://countermap.counter-strike.net/Nemesis/index.php?c=76#p76
As Griffin says, be very cautious with open terrain as it easily leads to high poly counts and a very slow VIS process. Movement on such terrain is also a bit strange sometimes as the player seems to get obstructed by the small edges between triangles sometimes. Personally, I create terrain like this manually in Hammer. More work but IMO a better overview of what's happening.