With one exception the cases decided in the federal courts under the Federal Rules of Civil Procedure have held that the plaintiff is free to plead a counterclaim in the reply, although in every instance the counterclaim in the reply under consideration was one related to the subject matter of the counterclaim pleaded in the defendant's answer.
Elizabeth G. Brown,
FEDERAL PROCEDURE - COUNTERCLAIM TO A COUNTERCLAIM UNDER THE FEDERAL RULES,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol52/iss8/5