Yes. You will need to use mod_rewrite inside an .htaccess file. Rename your CodeIgniter index.php file to ci_index.php, then use mod_rewrite to create rewrite rules that get routed to ci_index.php if they are routes that should be for CI. Anything else goes to WordPress.
I've done this successfully a number of times. For example:
Code:
# URLs routed to CodeIgniter
RewriteRule ^accounts$ /ci_index.php [L]
RewriteRule ^administration/.*$ /ci_index.php [L]
# No need to allow access to the following files
RewriteRule ^(wp-admin/install.php|xmlrpc.php|wp-trackback.php|wp-mail.php|wp-signup.php|wp-links-opml.php|wp-settings.php|wp-config.php|wp-load.php|wp-cron.php|wp-blog-header.php|wp-activate.php) - [F,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* index.php/$0 [PT,L]
In this example, index.php is WordPress' index.php.
This is actually really handy, because WordPress is lame when it comes to creating custom forms and ecommerce sites.
