Hi everybody, thank's a lot for your answers!
I can't find any informations with google, that's why i'm here with you, and i think it will be very useful for me!
I never been to Pakistan but i travelled many times in india with motorbike.
My first experience was 6 years ago (i was 20 - probably travel of my life) when I bought 350cc enfield in Delhi with the intention to cross Pakistan-Iran-Turquie and Europe to go to France.
Unfortunetely, because i was foreigner everything was difficult. i couldn't have all the necessary bike's documents (i think papers was fake...) to cross border and was force to stay in India. So I rode 2 months there, then i sent my bike home by boat.
During this trip, I "felt in love" with that way of travel and almost every years i go back to india, rent a motorbike and discover new parts of the country.
This summer i want to visit north of Pakistan (6 years after the first attempt:)).
For this trip, I do want to be in legality! Because, i'm travelling alone, I don't want any problem and i want to take a lot's of advantage of my holidays!
I understand that the problem seems to be the same that i had in india. I can't have a bike with papers at my name and i think it will be very difficult for me to do all the administrative procedure, alone in a unknown country.
After reading you, I think i can't buy new bike, but maybe i can buy second hand bike and keep the owner registration papers if he writes a letter saying that he lends me the bike. At the end of my trip i can sell him back the bike at good price or leave the bike somewhere...
Could it be legal for me to ride with lending bike?
Concerning the NOC, how can I obtain such a thing?
I don't want to loose my time and my energy (especially for impossible things
) when I will arrive in Pakistan, that's why i need to prepare the best I can this travel.
thank you again for your help 
(and sorry if my english hurts you..)