Changing the last commit message
What you are wanting to do is change (rewrite) the last commit message to add the signed-off line.
Go to the git status window (a common key binding is C-x g)
type c to get into commit
type -s to enable the signed-off line
type w to reword the last commit message, check that you have the wanted signed-off field.
type C-cC-c to do the commit
To enable -s as the default
You can set the -s option as the default by
type c to go to commit
If needed, type C-t to enable the common options
type -s to toggle the signed-off, toggle other options as desired.
type C-cC-c to set the defaults.