c_schmitz

Here is a golden bullet SQL query that does the mentioned trick:

INSERT INTO wp_gp_translations (original_id,translation_set_id,translation_0,user_id,status,date_added)
SELECT u.original_id,<dest>,u.translation_0,u.user_id,'fuzzy',NOW()
FROM wp_gp_originals o
INNER JOIN wp_gp_translations u ON u.original_id=o.id
WHERE o.status IN ('+active') AND project_id=2 AND o.id NOT IN (
SELECT original_id
FROM wp_gp_translations t
WHERE translation_set_id=<dest> AND (t.status='current' or t.status='fuzzy')) AND u.translation_set_id=<src> AND u.status='current'

Replace <dest> with your destination translation ID, and <src> with your source translation ID.
All copied translations are marked as fuzzy. Translation will not be copied if there is a fuzzy translation already.