Is VertiGIS Studio Reporting extensible? Can I extend it using custom code?

Yes. VertiGIS Studio Reporting includes an expression editor that can be used to create custom fields and expressions. In on-premises installations of VertiGIS Studio Reporting, you can include C# scripts in your reports and associate them with report elements (e.g. Report Header, Page Footer) and events (e.g. Before Print, After Print). More information is available in our documentation center: Use Scripts in Reports.