question

can u set a digital pin as a ground????

whenever you set a digital pin to low the chip is sinking current to ground, though the usual current rules apply per pin and for the chip total (which is much less than per pin all added together)

ground is such a simple thing, why not twist some wires together to make a Y

Look through the tutorials. You'll notice everything is tied into the same ground (completes the circuit).

What exactly are you trying to do?

Details, details....

7 segment

Yes, you can run a 7 Segment Display by sinking setting a LOW on the Digital Pin, but just remember your resistors either on the 7 Segment feed or the pins them self.