I tried converting some sky textures from hl2 to hl1 but the "ft" image is different "greyer" than the rest of them. So I guess it can't really be done.
The real problem though was that when playtested in hl1 the borders have a thin white edge. Someone else has had the problem on another site but no one answered. He even resaved a halflife sky texture and the borders appeared so I fear it is photoshop? I simply resized the images in photoshop and saved them as 32/24 bit tga.
Is there anything special that must be done with making sky textures to avoid this?