There is no option in Ghost to convert a GHO file to a WIM file. The only way to do it is to write the GHO file to a hard disk, and then run imagex to image the hard disk to a WIM file. Since imagex is part of the WAIK, you could add it to your GSS WinPE boot disk and in future create both GHO and WIM images from your master machine. WIM seems to create substantially smaller images as it explicitly excludes certain content from the WIM image, such as the swap file and various other system areas that are recreated each time the O/S boots.