For the conversion you only need the base game.
The conversion system is not Empires faction ---- FoG2 army list, its Empire's unit -- designated FoG2 unit and all that are needed are in the base game. Having said that, the dlc add a lot of interesting armies that are not in the base but buy as you wish to play FoG2.
The conversion system is fully moddable.
It also takes similar Empire's units and fits them to something suitable. So as an eg the relatively generic city militia in Empires can be a rabble, poorly trained hoplites or something else in FoG2.