PlanMaker spreadsheets (.pmd) can be exported as Excel spreadsheets (.xls) using the default File ► Save as.. function in the program. This is pretty much the only direct pmd to xls conversion we know of.
However, as pmd file suffix is used for several different file types, it may be possible that also other .pmd files can be somehow exported as Excel spreadsheets. In any case, several online converters advertise this conversion as well, but the results are questionable.