In order to add a filter to a template, you would need to apply the template to a schedule, create a new template from the schedule, and be sure to apply the correct filters to that new template. It would be much better to be able to add a filter to a template to change the people that the template will apply to.

As an example, if a regional team creates a template that does not include a filter for their team or site, then when they apply it all the people that have blank schedules from other teams/sites will wipe out those schedules inadvertently. It would be nice to then just add the filter that was forgotten when the template was originally created.