I think you should do the migration and manage SEO simultaneously. First of all, before moving your website you should enable WP SEO URLs. Also, run your site migration when the traffic index is the lowest. For SEO- and data-save migration procedure, i prefer automated converter http://www.cms2cms.com/supported-cms/wordpress/ It migrates data from your existing CMS to WordPress preserving SEO urls, internal navigation, categories, tags. Also, you'll be provided with the 301 redirection rules - just a line of code that should be pasted into the config file of WP site.
All in all, you may try free demo migration to see how the procedure looks like.