805 ILCS 405/6

If a person or persons required to register hereunder do not do so and such persons, or any of them, are unknown, civil actions may be brought against such person or persons, by filing suit against the business under its assumed name, naming all known owners, and designating as "unknown owner or owners" the other person or persons transacting business under such assumed name. A judgment rendered pursuant to this section shall be a personal judgment against all named owners and shall, in addition, be immediately enforceable against the property of the business and constitute a lien upon real estate held in the name of the business. At any time prior to satisfaction of judgment, if any, if the identity of a previously unknown owner is discovered, such person shall be named as a party defendant by motion to the court in which such case is pending or in which the judgment was entered, supported by an affidavit showing that notice of such motion has been given in the manner required for service of process upon an individual defendant by the Civil Practice Law, as heretofore or hereafter amended. Within 10 days after service of such notice, the previously unknown owner may appear and defend as in other civil cases; however, such defense shall not affect any judgment against any other person previously entered in the suit. If the defendant does not file an appearance within such 10 day period or if after hearing the defendant is found to be liable, the judgment, if any, theretofore entered in such suit shall be amended to include such person and shall at such time become a personal judgment against such person as if he had been individually named in the original complaint. (Source: P.A. 82-783.)