Your English is a lot better than my french
In theory you should be able to remove that trimmer pot you have labelled and just replace it with a full sized potentionmeter to give a pitch control. You will have to experiment with the value of the new pot but around 470K would be a good starting point. You may have to go a bit lower or possibly higher to get the best range.
The new pot should be wired just like the old trimmer. The bottom two points of where trimmer has been removed should be wired to the left and right pins of the new pot and the top point should be wired to the middle pin.