Wis. Stat. § 812.17
When the answer of the garnishee discloses that any 3rd person claims the debt or property in the garnishee's hands and the name and residence of such claimant the court may order that such claimant be impleaded as a defendant in the garnishment action and that notice thereof, setting forth the facts, with a copy of such order and answer be served upon the 3rd-person claimant, and that after such service is made the garnishee may pay or deliver to the officer or the clerk such debt or property and have a receipt therefor, which shall be a complete discharge from all liability for the amount so paid or property so delivered. Such notice shall be served as required for service of a summons. Upon such service being made such claimant shall be deemed a defendant in the garnishee action, and within 20 days shall answer setting forth the claimant's claim or any defense that the garnishee might have made.
Wis. Stat. § 812.17
This section allows a 3rd party claiming an interest in property in the garnishee's possession to be impleaded in the garnishment action. Following impleader, the statute requires the 3rd-party claimant to file an answer setting forth its claim to the property. The impleaded party is not required to file a garnishment summons and complaint in order to assert its interest in the property. This section is not limited to situations in which the garnishee's answer discloses a third party's claim to property in the garnishee's possession. Prince Corporation v. Vandenberg, 2015 WI App 55, 364 Wis. 2d 457, 868 N.W.2d 599, 14-2097.