Hey Marco, there's not currently a way to do that outside of a custom field with a list of all your users in the dropdown. That would be problematic to manage, though, as it wouldn't auto-populate with all users in that project.
I think I've seen this request before though -- you might head over to Agile Central Idea Manager (
https://ideas.rallydev.com) to see if this idea already exists, and vote for it! Also add your own use case to the discussion. If one doesn't already exist, I recommend submitting one! Thanks!