You can use a row generator, e.g. this will generate version IDs 11 to 15 for your query, so that your merge can insert/update them as required:
MERGE INTO product a
USING (SELECT product.product_id,
q.version_id
FROM product
,(SELECT ROWNUM+10 version_id FROM dual CONNECT BY LEVEL <= 5) q
WHERE product_id = 1234
) b
ON (a.product_id = b.product_id AND a.version_id = b.version_id)
WHEN MATCHED THEN
[UPDATE product]
WHEN NOT MATCHED THEN
[INSERT INTO product]