As far as I'm aware .XAPKs are not official APK extensions and therefore would need to be processed and installed by a separate XAPK installer. I find it highly unlikely that Package Studio would support .XAPKs I don't particularly consider that a feature gap either given that they are unofficial bundles usually found on app mirror sites.
Thanks a lot Matt, didn't know that.
I thought that its a special format. So i'll look for the right (really official) apk.