If you want McAllister or Alexander, you better make it a package with another RB, probably Dillon.
I do not see you getting McAllister or Alexander for Trent Green straight up. There is just no way, unless the person you're dealing with, is a complete moron.
The way I see it is, your RBs are fine, you really don't need to get anyone else. Tomlinson/Dillon is a good combo, IMO.