Anywho, go and grab Yob'Gul's file converter from the tools section, along with WC3 Viewer. Yob'Gul's converts, WC3 Viewer deals with extraction and can convert things to .blp
Then grab the Oinkertools for vertex editing, because Gmax is really really buggy with WC3 modding and, well, the Oinkertools were designed for it. Its in the sticky topic dealing with this very topic