Config/UDKinput.ini put at the end this line of code and choose what key(mine is ThumbMouseButton2) you want use for bind the zoom, change first FOV value to how much want zoom in and change the second value to your basic FOV:
Bindings=(Name="ThumbMouseButton2",Command="fov 60 | onrelease fov 120",Control=False,Shift=False,Alt=False,bIgnoreCtrl=False,bIgnoreShift=False,bIgnoreAlt=False)
https://www.youtube.com/watch?v=LEpqPeyU1PA&feature=youtu.be