It is possible, but you need a smart phone or some kind of technology with the MDE app on it in order to do it. I only point this out because my android is not compatible with MDE (for some unknown reason), so I cannot do it, but DH can with his phone.
Even then, you could go to the WDW site on your browser to add #4.
OP: The only way you would have an issue is if a member of your traveling party bailed after DHS *and* handed you their MB, saying, "Hey, I'm not gonna use my FoP, you can have it." You'd ride with your own - green Mickey - go back with theirs - blue Mickey. Maybe angry MDE. Nobody knows. As long as you scan into AK before using your FPs, you're golden.