Similar to what loki100 says. Play your Great Britain turn, then prior to ending use the Switch Faction command to China (CHI). Save and run the turn. The turn will run as Great Britain (Union Jack still in top left corner), but then switch to China during the movement phase. When the new turn loads you can play around with units and the like on screen, but do not use any of the F1 to F11 screens nor change any commodities on the (B)alance and trade screens, as it runs slowly and jerkily for this first turn only. Also, if you cannot see anything on the map (i.e. black screen) then use the RevealTI command. What I do is just do what I have to do with any units, then save and exit.
When you go to the load game screen China will not be an option to choose; don't panic. Load Great Britain, play, save and run the turn as normal. Next turn exit, and go to the load game screen where China should now be listed with Great Britain to play. From then onward each turn you will need to load each nation individually from the load game screen, play and save. You can run subsequent turns as any of your controlled nations, but you will always have to load each faction individually every turn.
I have done this successfully with emergent nations (Taiping, Confederate States of America, & Rumania), and those already at the start of the 1850GC further into the game (Greece). You can give them back to the AI control at anytime by just not playing them for a turn. You will be prompted to tick yes or no to letting a nation be played by the AI if you don't save a turn with them. I did this to the Taiping when China had almost completely defeated them. Currently I control 14 nations in my game, just so the economy runs and no-one is building mega-armies and conquering the globe......