We have found 2 software records in our database eligible for .cwk to .xls file format conversion.
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.
CWK
Files with cwk extension may be found as old documents made using now discontinued program for Mac called ClarisWorks.
XLS
Files with xls extension are best known as spreadsheets from Microsoft Excel. A spreadsheet is basically a grid of cells sorted in rows and columns. Each cell can contain text, numeric or formula data. Excel XLS spreadsheets can be opened in most other similar programs, such as OpenOffice.org Calc.