No. You can’t get more than 50:
If you have access to the data explorer plugin you can do a search like this to get all posts with the word:
SELECT p.id as post_id, user_id, topic_id from posts p
JOIN users u on u.id=user_id
where p.raw like '%WORD%'
and this to get them just for the user:
SELECT p.id as post_id, user_id, topic_id from posts p
JOIN users u on u.id=user_id
where p.raw like '%WORD%'
AND u.username='USERNAME'