It can be done...it just takes a bit of practice and some time to build up your pinky strength.
I'm with ninja tho... on that riff, it's easier to just use your ring finger.
Actually if you're doing all hammering and pull-offs, it's a whole lot easier and faster to tap it. Use your ring finger of the left hand on the 3rd fret, and tap on the 6th fret with your first finger, right hand.