Any way to replace to sky model?

I'm assuming that the sky in the game is another .mdl / .mdx, and that if I know the name I can modify it. Is this true? Would it be possible to make a sky of a different shape or size?

it's possible to make any kind of sky you want. search Jumbo_Pie's gallery, he made one or two.

I have made a dome sky with smaller domes for clouds, and a ring (from the game Halo) which worked in Warcraft III. So the answer to your question is yes.