A person who is not a party to an appeal may file in the court of appeals a petition to intervene in the appeal. A party may file a response to the petition within 11 days after service of the petition. The court may grant the petition upon a showing that the petitioner's interest meets the requirements of s. 803.09(1), (2), or (2m) .
Wis. Stat. § 809.13
A party who could have, but failed to, file a timely notice of appeal may not participate in the appeal as an intervenor or by filing a non-party brief. Weina v. Atlantic Mutual Insurance Co. 177 Wis. 2d 341, 501 N.W.2d 465 (Ct. App. 1993). A non-party to a circuit court action may intervene in an appeal brought by another party, even after the time for filing a notice of appeal has passed. City of Madison v. WERC, 2000 WI 39, 234 Wis. 2d 550, 610 N.W.2d 94, 99-0500.