Working for libvirt I had to add the Developer Certificate of Origin to several previous commits, where I forgot to directly use git commit --signoff. StackOverflow has that question, but it started mising -s for --signoff with -S for --gpg-sign.

At the end I used the following so sign the last 89 commits:

git config trailer.sign.key 'Signed-off-by'
SIGNOFF="sign: $(git config --get user.name) <$(git config --get user.email)>"
git filter-branch \
  --force \
  --msg-filter "git interpret-trailers --trailer '$SIGNOFF'" \
  @~89..@

See GIT filter-branch for other use cases of git filter-branch.

Only to find out that is as simple as

git rebase --signoff @~89