You have to use alpha channels (black is transparent, white is solid) and save as a 32bit .tga file.
Then you need to use rscripts, which are much like the shaders for quake3:
textures/alpha/support_grate
{
{
map textures/alpha/support_grate.tga
alphamask
}
}
That'll do an alpha mask, which is either transparent or solid. If you want to do varying levels of transparancy (grey in the alpha channel), you'll need to do blendfuncs, like:
{
map textures/alpha/scanline.tga
blendfunc GL_SRC_ALPHA GL_DST_ALPHA
scroll static 0 static 0.02
}
You prolly don't want the scrolling, but that's another feature you can use... for... i dunno what
You just put these in pball/scripts/maps/mapname.txt.