Not sure what you mean, but you could try modelling a sphere, then invert the polys, then apply the texture for your sky to the sphere. Then you'd place the sphere in the 3d skybox covering the little purple node thinger.
Just like the other sky textures I guess, just a textureset with the fight naming (skynamelt skynamedn skynameup etc) in vtf format and maybe with a special parameter in the vmt file.
Try to find out where Valve stored theirs and check those files out
I looked at Valve's VMTs, and it seems pretty much exactly the same as a normal texture, with these exceptions:
- the shader is "UnlitGeneric" as opposed to "LightmappedGeneric" (probably prevents the sky textures from being darkened)
- Valve used "$nofog" "1" and ignorez 1 (yes, without quotes); dunno why they'd do that...
- The naming convention is as Bluestrike pointed out: up, dn, lf, rt, ft, bk
Also, the VTEX documentation says that there's a $skybox parameter that goes in the texture's TXT file that forces the texture to be compiled as a skybox texture, but I haven't tried any of this, so I make no promises