Is there a way to create static extension function to existing class?

Only if the class had a companion object

fun String.Companion.two(string: String) = string + string

val s = String.two("foo")

Out of curiosity, what would be the purpose of creating a static extension function?

Doesn't pollute the autocompletion and works with classes not under your control

Just remember that if the class isn't under your control, it has to have a Companion defined already, or this won't compile.

:yes: https://twitter.com/kotlin/status/1194655130576019456