a playlist (m3u) file is really just a text file. Create a small one in subsonic. Then you can create one with a faster interface, like Winamp. You can just go through your Windows Explorer folder and right click on folders (albums or artists) and then choose "add to winamp queue". Once you've added a bunch that you want, you can save the playlist into the subsonic playlist folder, defined like GJ51 said.
then open them in a text editor, like notepad. Do a find and replace on all to make the winamp playlist point to the same folder as the one you created in subsonic.
does that make sense? that's how I made my big playlists..that take FOREVER to load.