First of all, sorry about my bad english.
I recently purchase a Canon 60d. I setup the camera for video shooting, lots of slowmotion. I want to set at 1280x720 50p 1/50 but minimum shutter sped is 1/60! It is possible to switch at 1/50 sec?

No, it's not possible.

But It's not necessary either - because for 50p you should be shooting 1/100th under most circumstances. If you were to shoot 50p at 1/50th and then slow it down to 50% in playback, there would be too much motion blur.