I would suggest installing a base Q2A first and checking the database schema to see where you need to import content into. You'd need to write a script to take the content from the XML file and write it to the database.
Probably need to do users first (qa_users table). Add the username (handle) and other details like email. If the user IDs in your XML file are numeric, insert them straight in, otherwise you'd need to make a list mapping the IDs from the XML to the IDs that get inserted into the DB.
Then the questions and answers and comments go into qa_posts. "type" will be 'Q' for questions, 'A' for answers, 'C' for comments. The other fields like title, content, tags etc should be fairly self-explanatory. Tags are stored as comma-separated values.
You can insert stuff straight into the database, then on the Admin > Stats page go through each of the recalculation buttons so everything gets counted and indexed properly. Feel free to post more questions here if you need help.