I've done it twice under regular game and VIP. On January 15th, 1870 or after, if you occupy Paris and hold it, an event will pop pretty quickly and every german nation except Austria will join you. I think there is a slight (5%) chance they won't, but I have yet to not get them all, except Austria. There are harder ways to do it but I don't know if you want to try them because the chances are harder. The game automatically declares peace between you and France and if you don't have the German states that France holds (Down in the Metz area) you get those, too.
In one game they had the North German Federation and the South German federation because I chose Yes on event that asked if I wanted more influence in Zollverein (?). But, the unification still worked and I got all the minors from both federations.