Q: I have a 2008 iMac how do I extend applecare

You have Applecare on a 6 year old computer?

https://discussions.apple.com/message/23871798#23871798

Vintage and obsolete products - http://support.apple.com/kb/HT1752 - "Vintage products are those that were discontinued more than five and less than seven years ago. Apple has discontinued hardware service for vintage products with the following exception:"

AppleCare cannot be extended past the initial 3 year period.