If you want to combine several mp3 playlists into one large list, you don’t need a special application:

Open the Winamp playlist editor, then drag and drop there all your playlists. Save the new, large, list with a new name.

If you have more than two or three playlists you’d like to join, you can avoid selecting and dropping them one by one:

Search for *.m3u files in the folder you have your music, then select all of the search results and drop them to the Winamp playlist editor.

Save your new list and sort it, randomize it, arrange it any way you like, and enjoy!