Right now, the profile for the user can only be imported when they 'connect' their LinkedIn account to your site. If an existing Joomla user clicks the "Login with LinkedIn" button, they will be able to login with their existing account and, at that time, import their LinkedIn profile information. So, it does work with existing users.
However, we don't have a method to let users import their information later on. That's something we're planning for the v2.0 release, due out in a few months. There will be a simply link "Import Profile from LinkedIn" you can place wherever you want on your site. Clicking it will import the user's LI profile for them.
Hope that explains,
Alex