Au cas où ça peut servir à d'autres, il faut bricoler ces requêtes SQL :
Pour les messages
INSERT INTO dc_post ( post_id, user_id, cat_id, post_dt, post_creadt, post_upddt, post_titre, post_chapo, post_content, post_notes, post_pub, post_selected, post_open_comment, post_open_tb, nb_comment, nb_trackback, post_lang ) SELECT nuke_stories.pn_sid, "alpinux" AS Expr8, 5 AS Expr9, nuke_stories.pn_time, nuke_stories.pn_time, nuke_stories.pn_time, nuke_stories.pn_title, nuke_stories.pn_hometext, nuke_stories.pn_bodytext, nuke_stories.pn_notes, 1 AS Expr1, 0 AS Expr2, 1 AS Expr3, 1 AS Expr4, nuke_stories.pn_comments, 0 AS Expr6, "fr" AS Expr7 FROM nuke_stories;

Pour les commentaires
INSERT INTO dc_comment ( post_id, comment_dt, comment_upddt, comment_auteur, comment_email, comment_site, comment_content, comment_ip, comment_pub, comment_trackback ) SELECT nuke_comments.pn_sid, nuke_comments.pn_date, nuke_comments.pn_date, nuke_comments.pn_name, nuke_comments.pn_email, nuke_comments.pn_url, nuke_comments.pn_comment, nuke_comments.pn_host_name, 1 AS Expr1, 0 AS Expr2 FROM nuke_comments;