is it possible to assign a "Receive"Task to a user, so he hasit in his task list, but can not complete it but the task will remain there until an external event happens ?
The receive task is not a user task (according to the spec) so it will not show up in a task list. Nor will it gain that ability in the forseeing future.
You can ofcourse create your own 'tasklist' by querying whatever resource you think is interesting and aggregate that
But what I want to have is a task in the Users Tasklist, that reminds him to do something in an external system, and the external system then signals Activiti that the Task has been performed and the process moves on.