If I follow correctly, the equivalent in K would be

    {,/(*t),/:t:$x}

Flatten (,/) the first element of the list (*t) joined with each element of the list (a,/:b), where t is the input x cast to a list of strings (t:$x). Curly braces make the expression an anonymous function, and since no argument names are specified explicitly the name x is the single input argument.