basically, I'm trying to display the voltage of the current going through a potentiometer onto a seediuno TFT v2 touch shield. I'm using a MEGA 2560 so i have analog pins available but not 5v or ground, so if an external voltage source is going through the potentiometer, can i still read it with the MEGA? i am new, sorry 
Probably a good example of where a picture is better than words 8) because I can't visualise what you mean...
That said, you aren't limited by a shortage of 5V or GND pins- they can branch, breadboard makes that easy with the power rails, basically giving you limitless connections to 5V and GND. See attached pic which I "borrowed" from the 'net. But make sure you don't draw too much current from the Arduino 5V: when in doubt, use an external source but have the GNDs all joined.
And as a by the way, this is conceptually incorrect.....
external voltage source is going through
.... since voltage is across, it doesn't go through. Current goes through.
