You can convert the .OBJ to .DAE (.DAE is an alternative to MDL0) by using a program called "FBX Converter". You can convert the .OBJ to .DAE in the program which allows you to use it in Brawl Box, the only problem i got from it was that I did not know how to assign bones for the model, but that's probably because I'm a beginner. So, maybe FBX Converter can help your situation. I hope this can help.