Probably no need to convert. The lossy DTS “core” is always a part of a DTS HD-MA track.
If you simply want to get rid of the lossless part, you can remux the file using MKVtoolnixGUI.
Drag the source file in, then select the DTS HD-MA audio track.
On the right side scroll down to “Audio properties”. Tick the checkbox “Reduce to core” and start the muxing procedure.
Another method would be to use ffmpeg (or its GUI XMedia Recode - Free All-in-One video and audio converter to convert the DTS HD-MA track to a lossy format. I recommend AAC stereo or Opus stereo.
If you want to retain the surround sound, you can also pick ac3 if you want.
I recommend creating just the audio track. Inserting it into your file is easier to understand if this process is kept as a separate step.
I personally use again MKVtoolnix for that kind of thing.
Just drag in your original file, and the converted audio track.
Tag, and name as required. Reorder the track if need be, to be before the lossless track.
That way it may get selected over the lossless track by the automatic track selection or by users who don’t care about it.
Here is an example of one I recently did. I left the lossy core DTS in there, and used the lossless in a separate step to mixdown to stereo and convert to AAC.
Optional (but very much recommended) when downmixing to stereo: analyze the mixed down track and apply normalization. That way you can avoid the “too quiet” issue on most clients. This can also be done during the mixdown+conversion in ffmpeg or XMedia Recode.
