How do I create a Clojure file in an existing Java package hierarchy?

I'm tryng to create a clojure project that interfaces with an Antlr4 parser. The parser generates Java files from a grammar file.  I have a build.xml that
1) generates the lexer and parser files, and
2) compiles the generated files.

The package name for the parser is

I now want to create clojure files in the namespace.

However, Idea won't let me do it. The only way I have found to be able to accomplish it is to touch a file in au/id/pbw/simplex; let's call it "junk". Then the package appears in the project, with the file "junk" and the package extension "grammar," containing the existing Java sources and the Antlr4 grammar file. Is there another way? This woudl seem to affect Java developers as well.