Since 2023 you could not import updates manually to WSUS. Microsoft offers you a script to download the updates from the update catalog when you provide the UpdateID to the script. The script defaults to localhost if you dont provide the WSUS server. For example:

.\ImportUpdateToWSUS.ps1 -UpdateId 12345678-90ab-cdef-1234-567890abcdef

But this script has a big issue for air-gapped enviroments – it still relies on the microsoft update catalog to download and import it to the WSUS server. But in the background the script just uses the ImportUpdateFromCatalogSite() powershell function. If you look at the parameters, you can parse the UpdateID but also “an array of the local paths where any files required by the update can be found.”