Can we use the pixhawk flight controller as the video transmission from the copter to the ground station Via ardupilot software.
That is not possible. The closest thing we have to that is OSD support to an external video transmitter. There are some systems that support telemetry over the same link as video, such as the SIYI HM30, but video cannot be processed or encapsulated within the telemetry stream by the autopilot itself.