It appears that until version 2003 Microsoft Excel software was capable of opening of .wk5 spreadsheet files and transform them to .xls workbooks. However, newer versions do not seem to support old Lotus 1-2-3 formats any longer.
If you don't have access to Microsoft Excel 2003, try to use for some open source alternatives, some of them may still support old Lotus 1-2-3 formats. Corel Quattro Pro might still allow wk5 to xls conversion for example.