I was told this by the ATT store here in Sacramento.
It can be added to your existing family plan, if you do the following (this is assuming you're adding it to the primary phone in your family plan):
1) make a non-iPhone number in your plan the primary number (if your name is not on that non-iPhone number, bring in the person whose name is on it). This way, your iPhone is now a secondary phone in your plan.
2) add the iPhone to your family plan. Now that the non-iPhone number is primary, your family plan stays unchanged.
3) Upgrade/activation fees still apply. Pay them.
After that, you're all set.
BL.