No problem,
run
git filter-branch --msg-filter "cat - && echo && echo 'Signed-off-by: Your Name <Your.Name@example.com>'" HEAD~2..HEAD
To sign-off the last two commits.
Then force to push them to the remote repo with the -f option:
git push -f origin
(Source: http://stackoverflow.com/questions/13043357/git-sign-off-previous-commits)