No
no
no
that's a TERRIBLE idea
Working out, and gaining weight will AFFECT YOUR GAME. You will be stronger, but it will take a LOT of time to adjust. You'll strike out where you usually would hit a line drive, you'll throw the ball over the first-baseman's head where you'd usually throw it straight to his glove.
If you had a year, then yes, you could possibly make this work. But in a month, you won't be able to gain more then a few pounds, and you'll screw up your game and not be able to adjust before the tournament. That's why professional players rarely do serious training during the season, they gain weight during the off-season and then just do cardio for the length of the season.