Hey, thanks for your anwers !
Your method seems very good, I also tried another one :
BiomeGenBase.biomeLIst[NUMBER] = new BiomeGenX...
it seemed to work, otherwise yours seems better, I think I will use it.
I didn't know about @Init, @PostInit..., but it seems useful too.
@Odai : use access transformers. these awesome things I learnt about thanks to Ebilkill are really, really, really useful.