The Importer does not support importing into an existing project. This was done on purpose so as to not open a potential security hole that would go along with "replace project with this name".

The only way to do something like that would be Import into dotProject as a "temp" project and then use the Template functionality to pull the tasks into your real project.

On the assignment front, simply change the names to the usernames of the relevant people. You can do this in Microsoft Project before export or during the Import process.