is it true that mentioned device doesn't support SNMP? One reason why you see these messages is when receiving flows from device which was added as ICMP into NPM and thus can't be queried via SNMP. But if it supports SNMP, the easiest solution is to change the node in NPM from ICMP to SNMP and discover any interfaces you are interested in. NTA will then monitor these interfaces automatically.
If the device indeed doesn't support SNMP, manually adding them to NTA via the link in the event is the only way for now. Interface speed is there just to calculate percentage of interface usage, in most cases it's safe to add 100mbit there.