Although you'll need to spend 100% of your time on your startup, at some point, you don't have to start that way. Just make sure that you do whatever you can to fully switch context when working at your job or on your startup. Trying to do both, switching back and forth during the day, will drive you crazy. So in short, yes it can be done.

BTW: if you haven't read it already, read "REWORK" by the guys from 37signals. They talk about this misconception.

Also, starting your startup this way is incredibly hard. But, if you are really serious, you'll find a way and all of the sudden you'll find yourself spending more time on your startup that at your job. Good luck.