As has already been said, you cannot build a portal in the End.
However, they can still be hacked in and used. Using creative mode and INVEdit, I located a naturally spawning End portal frame, and traveled to the End. From there, I placed a portal block and traveled to the Nether. Finally, I placed another portal block and was spawned back on the Overworld.
End Portal: (-1007,16,-123);
End Spawn: (100,53,0);
Nether Portal: (89,64,-21);
Nether Spawn: (11,62,-3);
Overworld Portal: (8,46,20);
Overworld Spawn: (74,64,163);
Note than when I spawned in the Nether, I was inside other blocks and no Overworld portal spawned with me. Spawning back to the overworld worked fine.