In a word, no. The external service would either need to be running meteor client code or it would need to manually make a DDP connection. I suspect you are looking to make a generic-looking HTTP endpoint that any server can connect to. If that's the case, your best bet at the moment is to use something like an API key.
- Distribute a unique key to each user/customer/service.
- Require the key in each HTTP request (server-side route).
- Hope that the keys are not misused/exposed, and program with that possibility in mind.