I do have insurance on my mogul. The sprint rep told me you are only allowed to add it with activation of a NEW phone, which make sense. If you do buy it used, just tell them it's new(they dont seem to care, plus it's new to you
But I would not keep insurance for more than 2 years, because...
$7/month x 24 months + $50(fee) = $218 which may be close the the cost of a new mogul without a contract in 2010. With a contract they may be much less. I'm sure by then there will be a better option anyway.