Mo. Rev. Stat. § 534.170: Amendments permitted and new parties added, when.

Any judge may, in open court and at any time, in furtherance of justice and on such terms as may be proper, on motion of either party, allow any complaint, summons, writ or other proceeding to be amended and permit new parties as coplaintiffs or codefendants to be added and correct a mistake in the name of either party.