If you are doing this to get France's population radically up, and growing at a fair rate constantly, then you could change a country's population growth rate by adding:
pop_growth_mod = x.xxxx
... to the .inc file, in whatever scenario folder it is, below where it says: "country = {".
If that line (pop_growth_mod = x.xxxx) is removed, then it will revert back to a normal growth rate of 1.0000. I believe France's is 0.9947 in the .inc file at the moment??. Also, x.xxxx is a number with 4 decimal places. The .inc files can be opened with notepad or similar.
If you need a step-by-step guide because you can't find the .inc files or whatever, then just ask.
If it is just a one-off increase then that probably won't help, but well... it's a lot easier than having to regularly stop and start a game and change the savegame files.