The subprocess call from the main process is not a External Web service call. They are internal invocations. You can pass input values to the subprocess from main process as need and use the input to pass it on to the header if there is a service step.
You may use the following approach.
Publish the sub-process.
Take the swagger file of it and create a service connector using the swagger file.
In the action binding tab you can add headers and publish the connector.
Create an app connection for the same and use it as a service step in the main process.