Mainly what you have to do is:

  1. Convert the site to WP
  2. Once all is good with WP edit .htaccess
  3. Above all the wordpress stuff you have to add this type of thing: Redirect 301 /myoldurl.html http://mydomain.com/newpagename
  4. You have to add that for each old file that you want to redirect to the new file
  5. There is info all over the net, including here: http://stackoverflow.com/questions/3696002/converting-a-static-site-to-a-wordpress-site-how-to-transfer-convert-seo-ranki