Can a rearend car accident cause cervical spinal stenosis and hyperextention of the neck?

No and yes: Spinal stenosis does not occur after an accident unless it is the aquired type due to a disc herniation, spine fracture or traumatic spinal instability or bleeding associated with it. Stenosis is developed due to degenerative changes and occasionally due to a small canal you were born with. Whiplash is a flexion/extension injury. With a stenotic canal, this force could injure the cord.

yes : If the force of your motor vehicle accident was sufficient to displace an intervertebral disc in your cervical spine, this may cause stenosis.