From Islamabad to Urumqi, there are flights available daily except on Fridays. Direct flights are accessible on Sundays and Wednesdays; while connecting flights are available on Mondays, Tuesdays, Wednesdays, Thursdays and Saturdays. For the sake of convenience, you can take the direct flight:
Flight CZ6008, 11:30 departs and 16:00 arrives at Urumqi. The flight is provided by China Southern Airlines.

From Islamabad to Kashi, there are only connecting flight on Sundays and Wednesdays with transfer at Urumqi.

China Southern Airlines has no office in Pakistan and you can buy the tickets from booking agencies in Mainland China. Or, you can submit your booking on this website and they will offer help.