Both the Apply Decals tool and the
infodecal entity it creates can only use textures from
decals.wad
. Any other textures won't work.
To create signs using textures from any other WAD, such as halflife.wad, you can instead create a very thin brush with the chosen texture and tie it to
func_detail.
Set the Passable (
zhlt_noclip) keyvalue to Yes (
1) and then the player won't be able to collide with it.