It's not possible.
All your planes MUST start their routes from the same airport.
But you can schedule "W-shaped" routes -- i.e. you fly somewhere, then somewhere else, and then return backwards.
So what you can do is fly all your planes from your Headquarters, to somewhere else, and then fly to a "focus city", and then reverse. Keep having your planes fly to the same 2nd destination over and over again, and on the map, it looks like you've got another hub, but it's sort of a 'fake' hub as the plane will always return to your Headquarters.
Attached is an image of my Route Map in the MT game. It appears that I'm running a hub out of both DFW and ORD, where I'm the largest airline at each location by far. But, really, my headquarters is Dallas. If you look carefully, every location I fly to ORD, I also fly to from DFW. This is because my planes are flying DFW-->XXX-->ORD, and then they reverse.