Kotlin for FRC is an unofficial companion extension to the WPILib extension. It adds various support features so that FRC teams can program effectively in Kotlin using Visual Studio Code.
Easily convert new Java projects created by WPILib to Kotlin ones
Quickly generate code files from templates just like WPILib does for C++ and Java
Simulate Kotlin code in the FRC Sim GUI
Any existing code in the project gets deleted when converting to a Kotlin project
This will never be fixed because Kotlin for FRC's conversion tool is meant to be used on a fresh project, not one already worked in. If you need a tool to convert Java code to Kotlin code, try try.kotlinlang.org's Convert from Java feature.