Thank you Peter and Helen,
Finally the option I chose was to make a backup of the forum activity in the original site, then to restore it in a standard course of the destination site.
After that, I move the posts of the forum directly updating the affected discussions in the mdl_forum_discussions table, from the restored forum to the existing site news in the front page of the destination site. I uptaded the course and forum fields.
This worked fine except... I tried to update the post's attached files in the mdl_files table. I thought that the only field to be changed would be the contextid, but there must be something else to do because although links to attached files appear inside the right posts, when I tried to download them, it fails (file not found)
Does anyone know what I forgot to do with the post's attached files in database?