Yea well theres no simple way of doing that... I've never opened one up but I'd assume it has a special sort of cable running through the neck which would not be easy to make longer.
Plus theres the issue of making a stand for the now loose screen....

Anyhow it would be far too complex to be worth it. Plus it would likely irreversibly damage the iMac...

I'd suggest you get a Mac Mini and run an LCD on it.