Beyond whether or not the csv module can handle NUL bytes, you might figure out if Excel will. Since the CSV format isn't some sort of "standard", its operational definition has always been what Excel will produce or consume.

I don't have Excel (not a Windows guy), but I do have Gnumeric and LibreOffice. I constructed a simple CSV file by hand which contains several NUL bytes to see what they would do. Gnumeric pops up a dialog and converts them to spaces (and then, oddly enough, doesn't think the file has been modified). LibreOffice didn't complain while loading the file, but when I saved it, it silently deleted the NULs.

I've attached the file should anyone like to experiment with other spreadsheets.