Is it possible to adjust the location/position of the brake pedal?

Yes, of Course you can. ALL motorcycles come with adjustable controls because it's important that the controls are adjusted to the individual. It's a Basic feature, like being able to adjust the seat forward and backward in a car to accommodate different leg lengths.

A quick google brings up the Honda Shadow forum, with details (and images) to help you with this.

http://www.hondashadow.net/forum/72-technical-disc...

Now, it's not moving it Forward, only up and down in an arc, but that's all you need to make it comfortable. Hope you'll take the time to get the clutch and brake levers on the bars in a good position, and the shifter in a good position, for you while you're at it. Takes only a few minutes and adds significantly to the comfort not to mention your reaction time when it counts.

You neglected to mention the year and model of bike you have but generally speaking, no, the rear brake pedal is not adjustable.

However, some models do have adjustable foot pegs so you can change the peg-to-pedal positioning.