Synopsis
You can use AspDotNetStorefrontCommon/AppLogic.cs methods named Custom_*. However, the code-behind already references AspDotNetStorefrontCommon, when trying to reference the startup function from AspDotNetStorefrontCommon is not allowed because of circular reference possibilities.
Procedure
Global.asax.cs is defined in the ASPDNSFApplication.dll and methods inside it must be appropriately handled.
The methods from Global.asax.cs can still be accessed via ASPDNSFCommon/AppLogic.cs. Search for "CUSTOMGLOBALLOGIC"...that region will have 7 custom methods for
Application_BeginRequest
Application_EndRequest
Application_Error
SessionEnd
SessionStart
ApplicationEnd
ApplicationStart