Thanks you all for your support in this.
Finally we use this sentence in order to allow fill a new attribute value to old items.
INSERT INTO ITEM_ATTRIBUTE
SELECT [OWNERID]
,[OWNERTYPE]
,<new item attribute id> as 'ATTRIBUTETYPEID'
,null as 'VALUE'
,[CREATEDBY]
,[CREATEDON]
,[MODIFIEDBY]
,[MODIFIEDON]
FROM [ITEM_ATTRIBUTE] WHERE [ATTRIBUTETYPEID] = <existing attribute type id> AND [OWNERTYPE] = 'I' AND [OWNERID] NOT IN (SELECT [OWNERID] FROM [ITEM_ATTRIBUTE] WHERE [ATTRIBUTETYPEID] = <new item attribute id>)
Where "<new item attribute type>" is the ID of the new attribute that has been added
And "<existing attribute type id>" is the ID of an attribute type that existed before for this item type
Do not forget to backup your database.
*Special mention to Melinda Skelton.
Best Regards,
John