There is nothing out of the box. There are lots of options though. For example, implement a mock version of the library that calls the external service and load it instead of the real thing by changing the (library) search path. You can also use wrap_predicate/4 to create a wrapper around any predicate. That is intended to call the original, but of course you don’t need to. You can even abolish/1 the old predicate and assert a new definition.
None of that is portable. Wrapping predicates is really SWI-Prolog specific. There is no portability in search path handling (but many systems have it) and ISO turned abolish/1 into something useless because you can only use it on dynamic code where, in addition to retractall/1, it removes the predicate properties (dynamic, etc.) SWI-Prolog kept the pre-ISO semantics where abolish/1 applies to any predicate.
Than you need an alternative implementation for the external server. I don’t know how much you can generalize that. I guess it can be anything.