Can I modify a terminal command to do additional stuff?
I'd tend to make a new command for this. I think it would even be logical to combine them into a single one.

go() {
    if [ -d "$1" ]; then
        cd "$1" &amp;&amp; ls
    else
        mkdir -p "$1" &amp;&amp; echo "Created directory $1" &amp;&amp; cd "$1"
    fi
}