You have a file with a list of items and you need to put them all in quotes. All but the last line needs to have a comma at the end.
Here is the list:
$ cat testfile.txt apple pear grape orange
Use sed to make the change:
sed -i 's/^/"/; $!s/$/",/; $s/$/"/' testfile.txt
Here is the list after:
$ cat testfile.txt "apple", "pear", "grape", "orange"