More flexible would be a double-quote-region widget which can be applied arbitrarily to any region of a command line:

    emulate -L zsh

    if [[ "$REGION_ACTIVE" -eq 0 ]]; then
        return
    fi

    if [[ "$MARK" -le "$CURSOR" ]]; then
        left_region_bound="$MARK"
        right_region_bound="$CURSOR"
    else
        left_region_bound="$CURSOR"
        right_region_bound="$MARK"
    fi
    length_region="$((right_region_bound - left_region_bound))"

    before_region="${BUFFER:0:$left_region_bound}"
    region="${BUFFER:$left_region_bound:$length_region}"
    after_region="${BUFFER:$right_region_bound}"

    quoted_region="${(qqq)region}"
    cursor_offset="$(($#quoted_region - $#region))"

    BUFFER="$before_region$quoted_region$after_region"
    CURSOR="$((CURSOR + cursor_offset))"
    REGION_ACTIVE=0

bind with eg

    zle -N double-quote-region
    bindkey '\e"'  double-quote-region