No, this is not possible. WSUS Offline Update only provides security-related updates for the currently installed Windows Version, either Windows 8 or Windows 8.1.
Greetings Gerby
Mach mit - der Übersichtlichkeit wegen! Füge Log-Auszüge als [Code] ein. Make it clear! Insert log excerpts as [Code].
Upgrade either through the store (non-Volume editions only), or by using an appropriate 8.1 installation media (matching your installed SKU/edition).
Microsoft update catalog: http://catalog.update.microsoft.com/v7/site/ Windows Install media download: https://support.microsoft.com/en-us/help/15088/windows-create-installation-media