Should you for whatever reason need to convert your Excel spreadsheets (.xls) to a format used in old Lotus 1-2-3 .wk1 files), the best course of action would be try some older (or better very old) versions of programs like Excel, Quattro Pro etc, because it is very likely that they once could save to the WK1 format, which will essentially achieve xls to wk1 conversion.
It is unlikely that latest versions of commonly available spreadsheet editors still support saving in this format.Open XLS file Open WK1 file
Spreadsheet program from OpenOffice.org productivity suite
A spreadsheet software application from Corel
Full-featured spreadsheet tool from LibreOffice suite
A popular spreadsheet application distributed with Microsoft Office suite