How to make slugs based on keyword?

Hello. The mission is to set post keyword while using xmlrpc and then make the guid based on it. So I’ve made changes to xmlrpc.php to set global variable to keyword, and then a plugin to hook ‘post_link’, which inserts keyword to the post_key field in the wp_posts (table altered) and changes the link it gets so that it contains keyword. On the main page everything is perfect, but when I try to open some post with ‘read more’, it returns 404. What do I have to change to fix it? Thanks a lot