So you have the iPhone in hand but its not yet activated? But you are planning on adding it to a family plan?
If so, set up a new line and get the free phone. Once you have that, you can use that number to activate the iPhone. When you run the registration through iTunes set it up as an exsisting number.
Then that free phone is yours to sell or do as you wish.
ALL new lines require a 2yr contract. If you add a line, it will require its own 2yr contract. If you roll those 2 lines into a family plan, you will re-start with a 2yr contract.
edit - re-reading your last post Im still confused. I will say though, that if you take a free phone from any wireless carrier you can bet your life that you will be locked into a 2yr deal. (although some do offer 1yr extensions)