This works for me:

cat file | sed -e 's/$/\n/'

Basically, use sed to add \n for each end of a line ($).

You can also do this, but the newline is adding before each line:

cat file | sed -e 's/^/\n/'

Basically, use sed to add \n for each beginning of a line (^).

Sed also accept file as input, so

sed -e 's/$/\n/' file # will do the same