Code:
$ cat t
col1,col2,col3,col4,col5,col6,col7
a1,a2,a3,a4,a5,a6,a7
b1,b2,b3,b4,b5,b6,b7
c1,c2,c3,c4,c5,c6,c7
d1,d2,d3,d4,d5,d6,d7
** Add string ",string" to end of each line with sed
$ sed 's/$/,string/' t
col1,col2,col3,col4,col5,col6,col7,string
a1,a2,a3,a4,a5,a6,a7,string
b1,b2,b3,b4,b5,b6,b7,string
c1,c2,c3,c4,c5,c6,c7,string
d1,d2,d3,d4,d5,d6,d7,string