How do you convert a 120v outlet into a 240v outlet?

I am setting up a pottery studio in a detached garage and need a 240v outlet for a kiln. The garage has 4 120v outlets that are all wired to the same breaker. I cannot add any breakers without having to run another power line and would like to avoid this. Is it possible to combine two 120v outlets to make one 240v outlet? Can this be done even if both outlets are connected to the same 120v breaker? There is also another breaker for the lighting if that helps. Thanks for any help you can give, I just want to have an idea of what I am getting into before I hire an electrician.