It seems like you're talking about a plugin architecture, not necessarily microservices. Microservices could be a way to implement it. Not all systems built with microservices could support a plugin like that, not all plugins need to use microservices, and even if you do you might have to write some form of adapter to integrate someone else's service with your system.

E.g. most video games support stuff like this, but they definitely don't use microservices.