Fonzies method is how i used to be done but there is a checkbox in Jed's HL MDL Viewer that lets you change textures to chrome / additive on the fly and then save them. Handy to have the program anyway seeing as it usually lets you check the appearance of the saved model as it would look in game.
Grab it:
HereOh yeah. And you need either the transparency DLLs for HL, or use Steam version. Also, Spirit of Half-Life supports transparency in models too.