A possible workaround involves disaggregating the single Page field information into two fields: Start Page and End Page. Then it would be a matter of modifying the affected reference type templates (Journal Article, Book Section, etc.) by renaming two of the Custom or other unused fields to: Start Page, End Page.
The next and final step invovles modifying the output style’s templates, replacing the Pages field appearing in the template with:
Start Page–End Page
The result will be to automatically have the em-dash appear between the Start Page number and the End Page number.