Recently I had to help with a site switch from the Joomla! CMS to WordPress. There is an tool published in the Worpdress website which can be found here:
http://codex.wordpress.org/Importing_Content#Joomla
http://www.onestopjoomla.com/extensions/migrate/joomla-to-wordpress-import-wizard/
but it doesn’t work for the new versions of these CMSes. There is an error that has to do with WordPress post_category field, which being obsolete for many versions,now has been removed.