Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.3)
// ----

@mixin extend-component($block, $elementModifier: '') {
  @extend #{$block};
  &#{$elementModifier} {
    @extend #{$block + $elementModifier};
  }
}

.block {
  background-color: blue;
  &__element {
    height: 20px;
  }
  &__element--modifier {
    color: red;
  }
}

.my-component {
  @include extend-component('.block');
  @include extend-component('.block', '__element');
  @include extend-component('.block', '__element--modifier');
}

SassMeister-output.css

.block, .my-component {
  background-color: blue;
}
.block__element, .my-component__element {
  height: 20px;
}
.block__element--modifier, .my-component__element--modifier {
  color: red;
}