It is possible to alter the crotch seam. If you took in the seam from the center front to the center back, it would lengthen the crotch. If the front crotch length is ok, and it was just the back that was the issue, don't take in much of the front seam, just take in the back and blend into the front.
Buy the pants LONG, because this alteration will shorten the leg length a bit. It will also cause you to loose a bit of the hip width.
To shorten a too-long crotch, you would alter the crotch from leg inseam to leg inseam... but that alteration will narrow the legs of the pants somewhat... and lengthen the legs.