Currently, the only known method for making war2bnet customs is to either directly edit your War2Dat.mpq file, or create a new, fill in MPQ, called War2Patch.mpq. When WarcraftIIBne.exe is run, it loads your War2Dat.mpq then checks for War2Patch.mpq, when the patchmpq is found, it will load the new data contained in it over the data for War2Dat.mpq. When making a War2bnet edition custom, you are basically creating your own "Patch" to load into Warcraft2.