Use the "Add data type" option from Dev Studio's "Data Types" explorer. Select "Existing data type" from the first screen of the wizard to choose any existing class to add to your application's Data Types.

You may be aware that this adds the class to the "Cases & data" tab of the current Application rule, which is what drives the "Data Types" explorer in Dev Studio. Now, this wizard additionally creates a Rule-DataObject instance to drive the Data Objects listing in App Studio.

Discussion on this topic was sought from the LSA Data Excellence (Pega 8.4) webinar conducted in July 2020. The webinar and its full set of discussions that arose from it are available at LSA Data Excellence: Webinar, Questions & Answers.