The best way I have found to do this is with a combination of fprintf and dlmwrite. While csvwrite is nice for writing out the data, it does not work well with writing text.
The basic form of what you are trying to do should look something like the following:
fo = fopen('fileout.csv');
fprintf(fo,'%c',string);
fclose(fo);
dlmwrite('fileout.csv',data,'-append');
There are some newer commands you can use in place of dlmwrite, and you can replace fprintf with sprintf or similar, but the basic concept is the same.
