I wrote a small command line util for converting a single player save to a hotseat game. To limit the use for cheating, it will only work for a game from the first turn of a game, and only if there is no admin password, and the no cheat flag is not set.
unpack the zip file into a dir of your choice (you need the zlib1.dll there too)
Create a single player save you want to convert, take note of what number in list the civs you want to make playable are.. first choice on list is number 0, and is what is normally played by humans except in scenarios.
start a command line shell (Start -> Run -> cmd )
cd to the dir you made and run:
hotseat "path to savefile you want to convert" list of civ numbers to make player playable
Eg:
hotseat "c:\civ4\saves\mysave 4000BC.Civ4SavedGame" 1 5
this will convert the 2nd and 6th civ in the list into human playable (1st one is number 0).. the numbers can be from 0 to 18, where 18 is barbarians.. I have no clue how playing as barbarians will be like
the converted file will be outputted as
hotseat.Civ4SavedGame in the same dir as the hotseat.exe, so you will have to move this file to the hotseat save game folder and you prolly want to rename it too afterwards.
Edit:
Get the util from this thread instead:
http://forums.civfanatics.com/showthread.php?t=153759