The only xar to xls conversion that comes to mind is potential export or recovery of Excel's autorecovery backup files (.xar) back to the original spreadsheet (.xls). Of course this is doable only with Excel, but what the Autobackup add-in does, is that it saves copies of the spreadsheet in selected internals, meaning you can just rename it to xls and work with that.
It is worth mentioning that Excel should delete .xar files the moment you save the spreadsheet manually, but you can often find leftover .xar files (hidden) in the \AppData\Roaming\Microsoft\Excel folder.