You should be able to drop in a Kotlin file into any Java project, and mix and match as you choose, without having to restructure the entire project, and move to a different build system.
You still need to build it with a Kotlin compiler though, right? Assuming you install the Scala compiler directly, rather than relying on SBT, you can do the same.
I tested it out a bit, it seems that first, the Kotlin files get compiled with the Kotlin compiler jars; after that the Java files get compiled with javac.
