I didn't have an opportunity to test this, but I believe that if you add the "Switch Workflow to Async" component just before your "Add Process Message" then it will resolve your problem.
One word of caution, this will not be a valid solution if you are calling this process as a webservice from a seperate process. In that instance, once you switch to Async it will return control back to the calling process, even though the current process has not finished.