Wis. Stat. § 846.09
In any action for the foreclosure of a mortgage, at any time after judgment and before a sale pursuant thereto, the plaintiff may be granted leave to amend the summons, complaint and all the proceedings in the action by making as defendant any person who is a proper or necessary party thereto. Such person so made a party shall be served with the summons in like manner as if originally a party, and may answer and defend, and all matters and proceedings as to the person shall be had and taken in like manner as if the person had been originally made a party thereto. After such person has been thus made a party and served, and the person's rights adjudicated upon, the original judgment may be so amended as to bar and foreclose the person thereby, or to make any provisions in regard to the person's rights and interests in like manner as it could have done had the person been made originally a party.
Wis. Stat. § 846.09