iCSV=Import["test.csv"]
->{{"C1","C2","C3","C4","C5"},{1,5,9,13,17},{2,6,10,14,18},{3,7,11,15,19},{4,8,12,16,20}}
iCSV = Transpose@
  Append[Transpose[iCSV], Join[{"Sum"}, Total /@ Drop[iCSV, 1]]];
iCSV // MatrixForm
Export["test.csv",iCSV];