Can Data Pages for a Pega Class be used when I create a subclass of that Pega Class?

Yes, but something in the loading of those Data Pages needs to know to create instances of your subclass. 

Ideally, the Data Page load sequence would have anticipated this and make a Dynamic Class Referencing extension point available so that instances of the intended type are used at runtime. You may however need to specialize the Data Page and reference your subclass as the "Page object" to instruct Pega on what class of Page the Data Page should return at runtime (and therefore what class-specialized rules to run during load).

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.