Migrating from other CMS to WP – losing SEO juice?

You should use a sitemap module to generate a list of URLs created by XOOPs [e.g. xSitemap].

Then you have to set up your .htaccess to 301 redirect each URL to its corresponding page in the new WordPress instance.

Finally, you should consider installing a WordPress plugin like Redirection to check for 404s and redirect them nicely to the new instance.