Adam D. Ruppe:
It just wasn't there but there's some people who think it would be a good idea to add (including Walter I think) so it might be in later.
It increases the syntactic variance of D code, and people coming from C++ will go using it straight (and not just in the uncommon cases discussed by Manu).
Perhaps it could be added with an ugly syntax that discourages it usage in 99% of the cases, like:
void __external_class_method ClassName.methodName() {} Bye, bearophile