You would need to import the data into the mp_LetterSubscribe table, if the rows don't correspond to rows in the mp_Users table then you need to put 00000000-0000-0000-0000-000000000000 for the UserGuid, and you need to correctly populate the guid for SiteGuid and LetterInfoGuid

Tools to import data are not part of mojoPortal, but it can be done with SQL Management Studio

Hope it helps,

Joe