It might be possible that your *._xls files are just renamed Excel spreadsheets. If that is the case, you can just rename it to XLS, open in Excel and work with it like you wish. In order to get XLSB workbook, you just need to re-save it using the File → Save as.. function.
Convert Microsoft Excel workbook to Microsoft Excel binary workbook file.
We have found 2 software records in our database eligible for ._xls to .xlsb file format conversion.
Microsoft Windows software - convert _xls to xlsb on Windows
A popular spreadsheet application distributed with Microsoft Office suite
Convert from _xls file format
|Yes, Microsoft Excel supports _xls file conversion as a source file type.||Yes||Yes||_xls editor||Yes||No||No||No|
|Yes, Microsoft Excel supports xlsb file conversion as a target file type.||Yes||Yes||xlsb editor||Yes||No||No||No|
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned _xls to xlsb. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert _xls to xlsb on OS X
A popular spreadsheet application from the Microsoft Office suite