I have an iMac 20" mid-2007 and an external 23.6" 16:9 aspect display, I would like to mirror displays keeping a 16:9 aspect on the external display, eventually stretching the image of the built-in 8:5 aspect display.
Is it possible?
It is not possible. Apple explains well what happens when you mirror displays:
"Your Mac will attempt to match the closest resolution between the primary (or built-in) and secondary displays. The usable resolution is the closest matching resolution for both primary and secondary display. If you choose a resolution that is not supported on both displays, the resolution will return to the closest matching resolution."
What are you doing that your only choice is to mirror the displays?
AFAIK Apple will not distort a display. Instead it will preserve square pixels by letter-boxing or pillar-boxing one or both of the displays as necessary.