Could you give me an example of the calls being made? Are the external calls being made to a service that is not being monitored by the Java agent? If that is the case, unfortunately there isn't a way to start a new transaction. I can file a feature request for you on this if you like.
However, if you are able to install the Java agent on that service, and if there is one method for all the endpoints, you can annotate your method using the addCustomParameter API call, and then query Insights to retrieve the custom attributes.
Alternatively, if there are multiple methods for the endpoints, and you are able to install the Java agent on this service, you can make use of XML instrumentation to start a transaction on each method. Each of these will then show up as separate transactions in your APM dashboard.
I hope this helps! Let me know if you have additional questions.