I am waiting for the tool, that yeti45 and AnRu announced.
Till then: More easy than in playlisttweak - I don't need an extra directory.
I use this Hexeditor by rightclick to open the original Playlist from its place in Windowsexplorer over network.
converting of m3u
I open my .m3u playlist also by rightclick to another instance of dfhexeditor,
hit F2 to change mode to work on file,
search '\' and replace '/'
search '0d0a' and replace '00'
search 'my winpath' and replace '/tmp/hddmedia/HDD1/'
Select all (the modified m3u) and copy
change to the dfhexeditorwindow with the playlist
look for '.0.<Playlist' (means end of an empty list)
change '0' (empty) to '75' (the number of mp3) and
insert between '75.' and '<',
the new playlist is ready.
Save and enjoy :)
In some cases additional utf convertig might be necessary.
All Playlists are in the same (1) file, separated by >playlist> 'count of mp3'. The restriction of 99 mp3 seems not to belong to one playlist, but to all together :( I tried two of 75, only one played - I tried one 75 and one 4 and both played.