Hi FelipeNS ,
We do not take any responsibility for the below action! Only for experienced users!
Please, remember to make a backup of your database before trying this!!!
As noted above you'll need advanced knowledge to understand the following.
1. Since ATUM version 1.5 and to follow the philosophy of WC, we moved all our metadata to a new table called [db_prefix]atum_product_data.
2. Suppliers are stored in the database as Posts (atum_supplier post type).
3. The product's supplier is stored in the "suplier_id" field from the Atum product table.
So, once you've figured out the supplier id for your products, you can generate and execute a SQL file with as lines as you need like:
UPDATE [db_prefix]atum_product_data SET supplier_id = [my_supplier_id] WHERE product_id = [my_product_id];
And I repeat again, please don't do this if you don't know what you're doing!!
Hope this helps.