If you have photoshop, alpha out the sword. (can be done using the background eraser tool). Then save it as a .blp file using war3viewer of the .blp tools found in the tools section here.
Next, download the attachment from the models section, upload the skin(with proper paths) into your map, upload the attachment into your map, then attach the attachment using the "special effect- create special effect" trigger. The attachment point will be "left hand" or "right hand".
war3viewer can also give you the kin file paths if you dont already have them. -VGsatomi