Hey Jorito! (long time no see!!! 
)
I just tried to compile the ADPCM decoder/encoder linked by NYYRIKKi on Mac, and just making a one line change it compiled fine (just comment out the #include malloc.h line in adpcmb.c, and then the make file works fine).
This is Jorito 5 years from the future, with the same question. The tool that was linked by NYYRIKKI worked fine, especially with the fix for Mac that was mentioned by santi. I was able to convert samples from my Mac to Moonblaster just fine. It was a simple matter of saving the files as 16 bit mono, 11khz, encoding them with the tool and renaming the files to .mbs.
If you want to try this yourself, don't forget to boost the volumes of the samples a (fair) bit, because they will be pretty low in volume otherwise 