In Scala, how can I define a companion object for a class defined in Java?

Question

I'd like to add implicit conversions to Java classes generated by a modeling tool. So I want to add them to the companion object of those classes, so that the compiler automatically finds them. But I cannot add them in a separate file, because the companion has to be defined in the same file. Is there anything I can do about this?

Of course, I can define all my implicit conversions in another object and then bring it into scope, but this requires an extra import. Any other solution?