In a case your backup is bigger than 512Mb there is a workaround. It's not handled by the GUI and you must do it from command line. Just copy your backup file to /store/backupHost/inbound folder. You can see that soon after, system process the backup, which moves to /store/backup/ and creates an entry in the database. After you will see this backup on GUI and able to restore it. If for some reason backup would not process then QRadar moves it to store/backupHost/invalid.