Yes. Converting a file format for personal use on a device you own falls under fair use in most jurisdictions. Distributing the converted APK to others is illegal.
Because XAPKs often contain architecture-specific data (like files meant only for 64-bit processors), a generic conversion might lead to an APK that crashes on certain devices. Conclusion Xapk File To Apk Converter Online
In a matter of seconds, the online tool had successfully converted his Xapk file into an APK file, ready for installation on his Android device. Xapk File To Apk Converter Online