OEMizer with EXT support.
 

This is the OEMizer that is included with PPCKitchen. I just updated it to support saving to EXT packages as well. Check the first page for the option to convert to EXT. 8)

http://joojoobee666.ppckitchen.org/Tools/OEMizer.exe

Some of the other cool things this offers is the automated "rebase to \Windows\" option, ability to detect multiple files with the same name in the cab and correct, as well as some other goodies.