Once you have your TWIC downloaded, drag and drop onto your destination DB. You can use the "Sources" tab to list your Big DBs sources. (i.e. TWIC #) I also think, but not sure, that you get a one year download from Chessbase.
ChessBase experts wanted

Yes and no. The games are added, but they aren't totally available to all ChessBase functionality. For example, I have Big Database 2012. I've added all of the games from TWIC since 2012. If I search for Magnus Carlsen, I can find all of his games that I have, both in the original database and those I've added. However, try creating a Dossier on him, and you only get the games, percentages, openings, etc. from the original database. Does anyone know of a way to get ChessBase to fully recognize added games? Thanks. I have ChessBase 11, BTW.
I have a Chess Base question:
So I have Big Database and I'm wondering how to update it. For example, if I download TWIC games, is there anyway to add these games to Big Database?
Many thanks in advance!!!!