The sed command can be handy for small tasks. You could use a modification of the command below:
#> sed 's/\(.\{2\}\)/&,/; s/\(.\{6\}\)/&,/' file_name
Just change the number inside the brackets where you want the comma to go...
If you have a 3 line file with "1234567890" on each line the output would be:
12,345,67890
12,345,67890
12,345,67890