I don't understand why you need to convert. WA saves playlists in both formats. The ones you make and save to a folder(s) of your choice, are m3u. The copies of these that WA makes for inclusion in the media library are m3u8. The active playlist is saved in both formats.
Is this not the case on your system?
Win 7 Home Premium 64-bit SP1, WA Pro 22.214.171.12492 beta, cPro MPxi_remix skin, 5.1 speaker system