a quick and dirty way to do this is by running this ONCE:
global $post;
$posts = new WP_Query();
$posts->query(array('posts_per_page' => -1, 'post_type' => 'post'));
while ($posts->have_posts()):
$posts->the_post();
$post->post_content = wpautop($post->post_content); // replaces new line chars with <p>'s
wp_update_post($post); // updates the database
endwhile;
(didn’t test it)
But how are you exporting your posts in text form? Maybe you could apply the autop filter in your export routine