Migrating from Another System
The general idea is to export all the posts and comments from your old blog into a TXT or XML file and then modify that file to work with the Movable Type migration utility or the WordPress XML Importer. The XML importer can read the WordPress export format.
This forum post may help with some of the modifications that are needed.
Check the Importing/Conversion section of the Plugins Repository for other plugins as they become available.