For Star Alliance, Turkish , Thai and Air China fly into Karachi...
Qatar Airways is a partner of many of Star Alliance airlines like United etc... so you can also travel on Qatar but it has to be booked Round Trip ... No Mix and match airlines allowed...
For OneWorld, Cathay Pacific and Etihad/Gulf Air (for AA members) are the best bet ..Although CX does not have good connections from US West Coast usually 8-10 hour stopover from West Coast to KHI. Malaysian will be joining OW from next year..
For Skyteam members, Saudi Arabian airlines will be joining from next year.. For now Delta Skymiles members can redeem miles on Malaysian airlines...
I hope more international carriers start flights from Pakistan soon.....