Yes you can. To my knowledge no 3rd party app can do both 60 fps AND lock the exposure. You'll need to use the S6's own camera app.
Launch the default camera, start recording video and tap to focus. You'll get that bulb symbol on the screen, slide it up or down and it locks the exposure/focus right there. Goodbye to that "pumping" exposure effect, yei!

I Usually set the slider a tad downwards, S6 overexposes by default. Hope this helped
P.S. I'm using Marshmallow, not sure about previous versions. I wonder why this problem has not been addressed before?