As I couldn't find any example for this I am wondering if it is possible to create a Liferay Portlet which uses (pure) Kotlin instead of traditional Java.
Kotlin targets the JVM - so technically: Yes.
Will anybody be able to help you doing so, share their findings: Probably not.
Do common build processes exist? Not that I know.
And it'll probably make a difference if you write a portlet for Liferay 6.2 or for 7 - unless you're aiming at just the standard JSR286 portlet.
This is a little bit like PHP-portlets in a Java Portal. It can be done, but once you run into the first issue, you'll have a hard time finding anyone to help you identify (or fix) the problem.