Help with custom-convert.conf

Hi I am currently using roon to playthrough my squeezeboxes in the house and all works fine. I have a few wireless speakers to that I connect to LMS via the UPNP2lms plugin that Roon sees via airplay only so I want to send to these via shairtunes. Shairtunes looks like it sends wavs to the players, but these speakers cant handle pcm only mp3 or aac. I cant seem to get LMS to transcode the wav comming out from shairtunes to mp3. Would custom-convert.conf help here if so can someone please explain how to use it as I have read several threads and am none the wiser as to what to do.

Any help appreciated.

Thanks.