It's impossible without your own coding. Currently this module works only on nodes and comments, and this is hardcoded. Given that it is:
Development status: Maintenance fixes only
you can't hope maintainers will add functionality; they already told they will not.
If you are using Signatures for Forums for Drupal 7, you can try with modules working generically on Entities. As far as I remember Rules could be used for something like that. For Drupal 6, I'm not prepared to even speculate.