Yes, it's possible. One way to do that, is to add a new term to the FormCommand.Quit command.
Here you will find an example that is doing exactly that (and code below for your reference)
private static IFormBuilder<T> CreateCustomForm<T>() where T : class { var form = new FormBuilder<T>(); var command = form.Configuration.Commands[FormCommand.Quit]; var terms = command.Terms.ToList(); terms.Add("cancel"); command.Terms = terms.ToArray(); var templateAttribute = form.Configuration.Template(TemplateUsage.NotUnderstood); var patterns = templateAttribute.Patterns; patterns[0] += " Type *cancel* to quit or *help* if you want more information."; templateAttribute.Patterns = patterns; return form; }