Rider

An insurance rider rides on top of an already-established policy. Like...you put in a brand new swimming pool in your back yard and you need a rider to insure it against cracking, the equipment against exploding, and the fencing against...not working. So you add $18 a month for that swimming pool rider, which covers you for up to $15,000 in damages with a $3,000 deductible.

Somethiing like that, anyway. Easy. (Rider.)