Is it possible to shot lets say a troll completely surrounded on every side by orcs by shooting overtop of the orcs.

Sure, just throw In The Way -throw (p. 32 in Hobbit Rulebook): 1-3 you hit the orc, 4-6 you hit the troll.

There are different values for In The Way -throw if terrain is blocking the line of sight. You'll find them in the same page.