PlanMaker spreadsheets (.pmdx) can be easily exported as Excel spreadsheets (.xls, .xlsx) by using the default File ► Save as.. function in the program, which is pretty much what pmdx to xls conversion is Just keep in mind that some PlanMaker specific formatting might be lost with such export, especially since XLS format is old Excel format.
Planmaker files are not supported in other spreadsheet software, which means that the only viable option how to convert pmdx to xlsx lies most likely with using PlanMaker.