Spring Boot with Kotlin

Suppose you have a Kotlin Spring application and it contains a build.gradle file with this code:

allOpen {
    annotation("MyAllOpenAnnotation")
}

And suppose you have some classes. How can you make it and its members open by using an all-open plugin?

_______________
class MySimpleSpringClass {
    var someField: String = ""
    fun someFun() {}
}