I have to import partner tags from several places, but each time I execute an import, the previous tags get erased.

For example, if I import:

id	tags
base.res_partner_2	Agriculture
base.res_partner_13	Agriculture

... then it tags both partners as Agriculture. Then, if I import:

id	tags
base.res_partner_2	Favourite partners
base.res_partner_13	Favourite partners

... then it tags both partners as Favourite partners, but removes them from Agriculture. This is exactly the same result of importing:

id	tags
base.res_partner_2	Agriculture
base.res_partner_13	Agriculture
base.res_partner_2	Favourite partners
base.res_partner_13	Favourite partners

I assume that behind the scenes it is using method 6, but I need it to use method 4.

I need to have both tags, but I cannot import them comma-separated, because this would cause the deletion of any user-defined tags in production database, and that would be awful. Also because it would produce extremely long data in those cells, and OpenERP seems to have a limit of characters.

How can I make that?