How to append data or values to an existing .csv file??

hi..suppose i have a .csv file named csvfile.csv.And it has the values as follows:

23 45

69 84

48 78

12 34

so it has two colums.Now wat i need to do is to add values staring from the 3rd colum with out deleting the values in the 1st and 2nd colums..How can i do this...??Any one plz help..!!