So I recently found out that logging into TFM with an Extinction account was possible, other than it adding a "+" at the beginning of the name to prevent the overlap of the same names.
How does it work? I did try but I got nothing.
I use the Steam version of the game.

Hi Vluvzz.

The only way to login to an extintion.fr account on transformice is if the account was created before the database merge, which happened November 2013. Any accounts created after this date will not be able to login to transformice, as the database will not have your information.
If your account you are trying to login with was created before the database merge, you should be able to use your login + password from extinction.fr (except that you have to add a + to the username).

If you have any other questions, please use the Question and Answers thread.

Thanks!