Medically it is possible, but is quite a mission for very little success. Doctors can surgically lengthen the penis by cutting the ligaments which secure the penis to the pelvis. However this only makes the limp penis look longer, and makes very little difference when it is erect.
There is however a danger that the erect penis will be unstable and less-rooted to the scrotum. Research has shown that the most you can expect to gain is no more than 2cm in length.