Mass update posts to include in post_content for Membership

this is for use with membership plugin

i need to update the post_content of a couple thousand posts to include the more tag at a certain point in the content.

a mysql query like
UPDATE wp_posts SET post_content = replace(post_content, ‘OLD CONTENT’, ‘NEW CONTENT‘); would have worked nicely. but i don't have / want to anything to replace in the existing posts, just insert after .....say 50 words.

please a suggest a mysql query that could do this? Or maybe a wordpress function like wp_update_post() ?