Soften yogurt by letting it sit out for a few minutes or by microwaving for 20 seconds. Heat a large nonstick saute pan over medium-high heat. Add the berries and saute until just starting to soften, but not hot throughout. Transfer frozen yogurt to a large bowl and pour over berries (with liquid from bag). Gently swirl berries into yogurt, without completing mixing in.
Spoon mixture into bowls or tall glass and top with pistachio nuts (you can also refreeze the swirl before serving).