Announcing externally defined enums to the message compiler does not work, see the following:

enum Foo;  // declaration of an externally defined enum
class A {
  Foo foo; //Error: unknown type 'Foo' for field 'foo' in 'A'
}

Corrensponding test case: test/core/msg_enum_decl_1.test

OMNeT++ 6.1