There are probably several solution to this.
here is one solution:
First: add a field ("FieldName") to the document allowing the user to select the receipient.
Second: open the Workflow organization db and add a Job Propery with a name "Routing". The formula; "FieldName".
Third: Go to your process in the Architect and on the activity to be received, you choose the Job Property "Routing" in the owner section..
Hope this gives you an idea or maybe other ideas :-)