I'm just a language guy, so I only know the dubmed down version he tells me, but basically, the BWGreymon line already exists in the game because Yuuya, one of the other humans in the game, has him as his partner. He has been working to make that data available to the player by hacking in evolution requirements for them, more than that I don't really know. This appears to cause a problem somewhere though, because as soon as you have one of the hacked Digimon and open the menu, when it tries to display the evolution data, the game crashes. That's basically the extent of my knowledge, except that adding new Digimon is difficult primarily because we don't have the source code, and instead are having to mod the assembler written by the compiler.