In my current organization I saw a project, where there is no Maven. The build system is a couple of configuration files for Eclipse IDE and that's the only way to run it. I'm glad I didn't have to do anything around that.
IntelliJ even has tooling to get you started:
https://www.jetbrains.com/help/idea/convert-a-regular-projec...
It took a day or two to get kt over to Maven: basically create a Maven file, make sure everything were in the correct standard Java layout, then build and unzip the war file and compare it to the unzipped war file from the existing build process repeatedly while getting the last tweaks in place.