Convert PM6 to INDD

pm6 to indd conversion is most likely related to some kind of export between Adobe PageMaker or Adobe Indesign. Adobe InDesign might actually support some files from old PageMaker, but it is likely that only the latest version (.pmd) and not all the older ones. In any case, if InDesign does not support your PageMaker file and you do not own PageMaker to export the file to something that can be imported to InDesign, there is really now way how to convert pmd to indd.

But you can always try to contact Adobe support or community forums for help.