Alerta supports lot of severity levels. Below is a link for the full list.
http://docs.alerta.io/en/latest/api/alert.html#severity-table

As of today there is a hard mapping of Kapacitor's AertNode critical, warning and Info levels to on of the Alerta's severities.
There is no possibility of setting severity of alerts when sending to Alerta apart from what kapacitor already mapped.

This issue is being raised to request the support for setting the severity for alerts that are being sent to Alerta from Kapacitor.