As of Pega 8.5 there is an "Import from Spreadsheet" option available when adding new Case and Data Types to allow existing XLSX or CSV files containing existing business data to drive the new Case/Data Type's data model.

Be mindful that the spreadsheet provided may reflect the Client's physical data model. That is, it could have come from an export from the database tables in the Client's System of Record. It may not meet your Application's need to align its Data Classes with the existing System of Record structure. Your job as the LSA is to navigate this and proposed a justifiable design to support the needs of the Application to be delivered. 

If the import capability is desired, there is a step in this process that provides you an opportunity to rename the fields from the spreadsheet, so that the Data classes can reflect business-friendly names and labels, rather than those from the source of the spreadsheet which may reflect database naming conventions or constraints.

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.