For those who may be insteresting, I managed to do it with this tool called New Vegas New Game + It basically reset everything in your save (even NPCs, quests, locations, etc. from mods) exept your character and all the containers in Doc Mitchell's house, and start a new game just after you've finished the character creation with him. Keep in mind that even if the instructions of the mod say you need to be at Doc Mitchell's place to do it, the new game will start exactly at the same place your character was. So if you are using this to play a complete overhaul mod like me, I'd say it's better to do it at the place your new game should begin.