I was hoping that an expert might come up with some good advice here! Clearly, I think that closely coupling any service with the BPMS is a bad idea from a business architecture point of view. It will also add complexity to the conceptually simple file transfer operation because you probably need to make sure that the event signalled by the file transfer service has been processed ok before completing the file transfer service. I will be interested to know the benefits of doing it that way.

However, waiting on an event is what is happening throughout the Intalio BPMS operations when dealing with messages ... all you need to do is define the message and include it in your process flow as an intermediate message event. The source of the message is within a non-executable pool representing your external file transfer service. If you just design a simple process with an intermediate message event and start it, you will see the process wait for the intermediate message.

Intalio Designer will generate the WSDL that your external service needs to communicate with the business process.
You will need to use explicit correlation (so that the message goes to the right instance) ... there is a sample use of explicit correlation which may be a help.

David