can I create a compiler plugin which generates additional code based on existing code, but without needing an annotation on the classes it modifies?

Yes

sorry, let me rephrase. Where can I find example code for a compiler plugin that modifies generates additional code based on existing classes, but where the classes do not need an annotation on them? I was looking at https://github.com/google/ksp/blob/main/examples/playground/test-processor/src/main/kotlin/JavaBuilderProcessor.kt and it is annotation based

for example https://github.com/bnorm/kotlin-power-assert does that

You can do whatever you want with an IrElementTransformer