@monkey10 - You can always purchase the device and add it to an existing account, but your existing iPad plan may have to change as the data only plans are a bit different than the ones intended for cell phones. The plans that allow you to have up to 10 devices in one account are called the Share Everything Plans. Here is some more information for you to look over: http://bit.ly/xdDajD Here you will be able to tailor a plan that best fits your needs.
If you have any questions, please do not hesitate to ask!
Thank you and take care!
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