I honestly wouldn't. No one knows when Mendenhall will be back and Hankerson is, at best, the 4th option in the Skins passing game behind Garcon, Moss and Davis.
EDIT: Just saw Mendenhall might play week 1 (!?! holy moly fast recovery). In that case, I would probably drop Randle for Mendenhall. But I highly doubt he would be the feature back right out of the gate.
Hankerson is not a good option. I'm from DC and root for the skins, but I rarely like taking players from their team... especially not a 4th, maybe 5th option.
Mendenhall is a good call. Drop Randle. Again hes like a 4th option for the NYG and Mendenhall is RB1.
I would snatch up Mendenhall yesterday. He might not be back for a few weeks, but he's well worth the reward. He's a winning lottery ticket...better than Randles chances of being a fantasy starter and that's what you want out of those bottom of the roster spots.
I think Mendenhall's upside is much greater than Hankerson's. Early reports are that Mende is healing well and while he may have a slow start he's the most talented guy in their backfield.
If you can, drop Randle and pickup Mendenhall. He might take a few weeks, but his upside as the starting RB for Pitt is much higher than anything Randle will likely give you this year.
Aside from that, i would stay put. You have plenty of depth across the board to take a wait and see approach.