As long as your ONLINE WSUS server shows the Upgrades category selected and the correct products selected (see https://www.ajtek.ca/wsus/how-to-setup-manage-and-maintain-wsus-part-3-windows-as-a-service-waas-and-group-policy-administrative-templates/) then you should be able to easily export and import your Feature upgrades using the regular method (https://learn.microsoft.com/en-us/mem/configmgr/sum/get-started/synchronize-software-updates-disconnected)
The process shouldn't be difficult. It should be approve on the Online WSUS, wait for the content to download, then, run the export commands, copy the data over to the offline WSUS, run the import command, and allow your systems to recognize the new upgrades and approve them.