Is it possible to federate a remote 3rd party graph while extending types with peer subgraph types?

I understand that this is super specific, but here is what I am after:

                           /--(subgraph)--> Microservice 1
Apollo Federated Gateway -|---(subgraph)--> Microservice 2
                           \--(subgraph)--> Proxyservice 3 --(remote)--> 3rd Party Graph

So the above is already tricky enough, in terms of how to subgraph a normal remote graph (any pointers here much appreciated), but I would also like the Proxyservice to extend the 3rd party graph to mix in subgraph fields into the proxied 3rd party graph.

Is this even possible?

If still a bit abstract, consider Microservice 1 is a Product service and the proxied 3rd Party graph is a Cart API. I want to be able to mix in product fields into the returned cart types.

Now I have seen ways to do this sort of thing with @graphql-tools/stitch but my company is running a top level Apollo Federation Gateway, so I cant seem to make this work?

Any help appreciated