There doesn't seem to exit any converter available capable of converting old Claris documents (.cwk) to Microsoft Excel format (.xls), i.e. cwk to xls conversion. Your best bet is to access these .cwk files on some old Mac and save them to some common format like CSV, which you can then transfer to Microsoft Excel and save as XLS.
FileMaker Pro supposedly still supports old CWK format, so that could be another choice for CWK to XLS conversion.Open CWK file Open XLS file
A database management program for Mac
A program used to create and manage information databases