It's possible but you'd have to swap phones on that account. The ERIS would have to be activated once leaving the store but you could always swap the phone's once you leave if you want. That also means you can lose the data plan if you just wanted to have the Eris as a security backup phone.
I think I asked about that for the storm bogo. The VZW worker told me that I had to activate it then and there, and I couldnt keep it as a back up.
But i will check again.