Default lists

// ----
// Sass (v3.4.4)
// Compass (v1.0.1)
// ----

$default-list: (
  key1 : value1,
  key2 : value2
) !default;

$additional-list: (
  add1 : value3,
  add2 : value4
);

$list: join($default-list, $additional-list);

@each $key, $value in $list {
  .#{$key} { content: $value; }
}

SassMeister-output.css

.key1 {
  content: value1;
}

.key2 {
  content: value2;
}

.add1 {
  content: value3;
}

.add2 {
  content: value4;
}