Mo. Rev. Stat. § 509.510: Supplemental pleadings.

Upon motion of a party the court may, upon reasonable notice and upon such terms as are just, permit him to serve a supplemental pleading setting forth transactions or occurrences or events which have happened since the date of the pleading sought to be supplemented. If the court deems it advisable that the adverse party plead thereto, it shall so order, specifying the time therefor.