I lost my honda civic 2010 fn2 key i only have 1 remaining key. I have found the same key as mine but programmed to a same model car could i re-program the key to my car so i can start the car and open the doors etc obviously changing the blade as its already cut for other car.

 

yes, there are howto's on youtube for programming key fobs to your car, you would need to reprogram all at the same time

the same would apply to reprogramming the keys to the ecu for immobilizer purposes but only the dealer or a locksmith with an hds would be able to do this