Run Java to Kotlin converter from the command line?

It seems that it's possible, but there's no tool for that, just code. There are tests in j2k module in Kotlin Github repository, the example below is taken from the tests (AbstractJavaToKotlinConverterSingleFileTest):

private fun fileToKotlin(text: String, settings: ConverterSettings, project: Project): String {
    val file = createJavaFile(text)
    val converter = JavaToKotlinConverter(project, settings,
                                          IdeaReferenceSearcher, IdeaResolverForConverter)
    return converter.filesToKotlin(listOf(file), J2kPostProcessor(formatCode = true)).results.single()
}

I suppose, you can make a .jar of that module and write your own simple wrapper for JavaToKotlinConverter to make it work on files.

UPD: the code has migrated to the intellij-community repository, you can find relevant tests here: https://github.com/JetBrains/intellij-community/blob/2382372993bee233bc6fce95c3917a68a33da111/plugins/kotlin/j2k/new/tests/test/org/jetbrains/kotlin/nj2k/AbstractNewJavaToKotlinConverterSingleFileTest.kt