Yes, as long as you own the app or game. Converting your own legally downloaded XAPK for personal use is generally allowed. Redistributing converted APKs is illegal.
Converting XAPK to APK for apps you do not own (pirated/cracked software) violates copyright laws. Only use converters for apps you have legally obtained. Xapk File To Apk Converter Online
You cannot compress a 2GB game data file (OBB) into a 50MB APK file and expect it to install. The Android operating system has size limits and file structure rules that prevent this. Yes, as long as you own the app or game