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 xlsx conversion is Just keep in mind that some PlanMaker specific formatting might be lost with such export.
Considering that Planmaker files are not supported in other spreadsheet software, the viable option how to convert pmdx to xlsx is to use PlanMaker.