Maybe it won't be possible right away but in the future about 2-3 months down you willl likely be able to obtain a gevey or rsim for the S6 .
Try contacting the provider you are locked to, they will usually charge a $20 fee or so and will unlock it for you.
They would never do that, as the phone is in contract for 1-2 years and the company basically sold the phone at a cheaper price only to get revenue from the 2 year plan the customer opted for.