Just to be clear, I doubt you'll ever be able to convert a MIDI file to an ST sound file, whether it be SNDH or whatever.
I appreciate the concept, but remember a MIDI file is not an audio file in the first place, MIDI is a protocol which merely records the event from a sound source, key note, note length, velocity etc... it in no way contains sound. So as others have said here, the only possibilty is to connect your Atari ST as a sound source using a Chip Tracker to an external MIDI sequencer, and map those MIDI tracks to sounds on the tracker on your ST. Either way, your Atari sounds you wish will of course have to be the sound source as others have said here in various ways to achieve the result.
I am currently re-setting up my studio in a similar way, not to play existing MIDI files, but to utilise my ST and Falcon as a sound source in a MIDI set up.
Doing a search on the web helps, stumbled on this, which is a similar concept:http://teragonaudio.com/article/You-can ... files.html
Atari Falcon 14mb, 68882, Dual 8gb CF, Steinberg FDI & Analog 8
Atari Jaguar, Rotary controller, Skunkboard & Cat Box
Atari 520STFM 4mb, TOS 2.06 switcher, OverScan, GigaFile, PARCP-USB, Unicorn-USB, System Solutions MiniS HD, SyQuest drives, ICD Link II, PhatBoy MIDI Controller, Philip Rees 5M MIDI merge box, SoundPool MO4, Steinberg MIDEX, SMP II, Emagic Log 3, C-Lab Unitor 2, Combiner & Export expanders