The only way to produce .pmd spreadsheets from MS Excel sources (.xls, .xlsx) is to use PlanMaker. It appears the PMD format used in PlanMaker is not that well supported in other spreadsheet programs, perhaps with the exception of programs based on Ashmapoo software.
However, as several more file types use pmd extension, it may be possible that some other xlsx to pmd conversion exists. If you are looking for something specific, please write in the discussion below with as much as possible relevant information.