As a general matter, you can use the 'asmto8xv' tool to convert .asm and .inc files into Mimas format. In this case, mt3notes.inc uses #define macros instead of normal equates, so to create the file below, I had to change the #defines into equates.
As you probably know, Mimas doesn't currently support macros at all, so instead of this:
song:
playsection(startbit)
playsection(lastA)
playsection(startbit)
playsection(lastB)
endsong
startbit:
note(rest,rest, c3,f2, eighth)
note(f2,a2, c3,f2, eighth)
...
endsection
you'd need to write something like this:
song:
RORG 0
DW startbit
DW lastA
DW startbit
DW lastB
DW 0 ; end of song
startbit:
DB rest, rest, c3, f2
DW eighth
DB f2, a2, c3, f2
DW eighth
...
DW 0, 0, 0 ; end of section
I haven't tested this, but it should work. Hope this helps.