Sadly USB Loader GX doesn't support NKit yet. Somebody needs to hack the cIOS and few people know how to do that. For now you'll have to convert the .nkit file to .iso, then optionally to .wbfs. NKit has a UI now so I'm updating the Vault's NKit
page with the current 1.4 release.
Does USB Loader GX support NKit? That's the main loader i use for my hacked Wii to play games on.