I've picked out my headphones jack. Can i change my microphone jack to headphones jack only by soundsystem configuration? My soundcard is Intel82801I (ICH9 Family) HD.
It requires significantly different circuitry to drive speakers vs read an input signal (from a mic). That means the port would need to be multiplexed at a hardware level, not just in software. Because of this, it would add extra cost to the hardware. The extra cost combined with a very limited use case means you won't find many (if any) hardware devices that have that functionality.
However, if your hardware does support it, it will be in some documentation somewhere (which may or may not be public). So if you're really aching for it, you can try and dig though that. However, once you find that your hardware is capable, your driver must also support it. I'm not aware of any driver that does, but if it does, it is probably documented, so you can look there first.
If you have hardware that supports it, but your driver doesn't, you'll need to modify the driver to do what you need.