The format.signoff git config option enables adding a signoff on patches automatically, but I don't think it applies to commits in general just based on the name and the description.
The command line equivalent of what you want would be to set up a git alias that runs
commit -s for you along with getting used to using the alias instead of
git commit. You can set up custom git commands in Merge (i.e. call git with whatever argument you want) but I don't know how well that would play with the GUI (my guess would be not well).