What do the following five-letter words have in common?

ATONE
CANTO
ENACT
OCEAN

HINT:
Adding a single, different, letter to each word will change them all into the same six-letter word.

Answers to added letters:

ATONE + C
CANTO + E
ENACT + O
OCEAN + T

Answer to final word:

OCTANE

Note from JA: OCEAN is an anagram of CANOE, and OCTANE is an anagram of ACT ONE.