(1) Do you have an XML export file? If you do then you can import it using these instructions. http://en.support.wordpress.com/import/
(2) If there is no exporter and you cannot get an XML file of your content then provided there is an RSS feed on Letterdash Staff may be able to help you import the content using it. http://en.support.wordpress.com/contact/
(3) I'm not saying this is the case, so please do not choose to be offended, but if you are looking for the ability to autopost duplicate content to both sites then WordPress.com does not provide such tools as they are commonly used by content thieves who set up splogs.
see also > Google webmasters > Duplicate content > http://www.google.com/support/webmasters/bin/answer.py?answer=66359