Hi Jรฉrรฉmie,
We do not support this in the model interface. Indeed, we think that modifying the coefficients by hand is not the best way to go since they are optimised based on past data. I am not sure modifying them will actually increase your classifier performance.
If you really want to pursue this idea, it is actually possible via python. You would need to export the model to an ipython notebook, run it again and play with the coef_ attribute of your classifier.