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If I have a family plan with AT&T can I add an iPhone to that with the existing customer's option ($20 a month)?

Yes, that is exactly what I did. If the other phones on the family plan are not iPhones it doesn't matter. They just will share the features that will work on those phones, as far as I know.
 
If I have a family plan with AT&T can I add an iPhone to that with the existing customer's option ($20 a month)?

Yes! That's what I did. I have a family plan with two lines. Your plan doesn't change with the exception of the unlimited data plan and 200 text messages added to your iphone.
 
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