apologies for the delay in replying. I'm afraid this isn't possible, the 3rd party content will be pulled from the upstream server, regardless of whether MS updates are pulled from the upstream or Microsoft.
Basically this is how WSUS handles the content, I haven't seen an official explanation, but I assume it's because the content needs to be signed before it gets pushed in to the downstreams, and since this is done with locally published content at the upstream, any external sources wouldn't have that signing. Also, because content can be edited/created internally, there wouldn't be any external source that could host all such changes.
Makes sense. Appreciate the reply.