Ivory isn't bad. Certainly worth a 7th rounder, IMO.
Blount isn't anything special and he is too similar to Greene. I don't think adding him helps much at all.
Jackson is an interesting option, but I don't know how much he has left in the tank and he seems to have some nagging injury.
They should just let the guys they have play more and see what they can do. McKnight and Powell are young players who have not really had much of an opportunity to carry the ball. Both have looked capable in limited action.
I think as Jets fans too often look over the fence at other teams' players thinking the grass is greener. In this case, it isn't.
Ivory and Blount are average to below average NFL RBs... and not clearly better than the players currently on the roster.
McKnight, in particular, would be a successful player with a competent coaching staff. They would recognize his athleticism and ability to make explosive plays and they would find a way to put him in positions to succeed, not bench him for all but 3 offensive snaps a game because he can't pass block.