For the most part, transforming existing Java code to Kotlin is straightforward. IntelliJ and Android Studio can even transform Java to Kotlin automatically (look for "Java Kotlin Converter"). However, you will have to adjust the generated code -- sometimes to make it compile but almost always to use clean, idiomatic Kotlin code.
I'd recommend to use the converter as it gives you a head start. Plus, it's actually a great way to get started with Kotlin as well because you see how it transform your Java code into Kotlin.
But keep in mind to store your original Java files somewhere (e.g. version control) because they will be gone after you use the converter.