Pre-configured SNMP traps are contained in the /etc/alertd/alert.conf file. F5 does not recommend, or support, the addition or removal of traps or any other changes to the alert.conf file.
Custom, user-defined SNMP traps should be defined in the /config/user_alert.conf file.
Depending on what you what to monitor, you can add it to user_alert.conf, example:
alert BIGIP_LOG_WARNING ".*No members available for pool" {
snmptrap OID=".1.3.6.1.4.1.3375.2.4.0.201";
email toaddress="your.textemail@company.com"
fromaddress="root"
body="No members available for pool"
}