This is done fairly often for different alerting scenarios. Here would be an example:
Just edit the existing emailFilter.xml. Leave the templates alone, add in this filter rule, config the bottom of the file for the sender/retry time, then create the email alert handler in the Custom UI.
<FilterRule name="Classic_KPI_HT_BREACH-Alerts">
<Conditions>
<condition type="AlertType">CLASSIC</condition>
<condition type="AlertSubType">KPI_HT_BREACH</condition>
<condition type="Level">Critical</condition>
</Conditions>
<Addresses>
<sendTo type="email">test@domain.com</sendTo>
</Addresses>
</FilterRule>