The way to monitor unknown devices is to build your own probes. It is most likely that this companies routers were never added to the dude but you can quickly determine if SNMP is working by right clicking on the device and select SNMPWALK. If the screen fills up with entries I guarantee you can monitor these devices.
When you SNMPWALK any device that supports the generic DOD intenet MIB "iso.org.dod.internet.mgmt.mib-2.system.sysDescr.0" or "1.3.6.1.2.1.1.1.0" is the very first result. This is the system description and it will be text and not numbers since the DOD internet management MIB is pre-installed. With that in mind you will see numbers for any entry that you do not have a MIB installed for. Also you do not have to have the MIB installed but now that you have you should see the SNMP entries as text instead of digits. Since the MIB gives you text entries you should be able to search for things you are interested in by name like "temperature" also the MIB will allow the Dude to know what kind of MIB entry it is reading i.e. a number, or a gauge, or an octet string. This allows it to format the data correctly BUT you can still use MIB results without having the MIB by using the string directly.
Further down is private.enterprise where the MIB you installed will show what specific entries apply to that enterprise.. Here is Cisco's first entry which is teh boot loader description ... 1.3.6.1.4.1.9.2.1.1.0.
So you should be able to build your own CPU probe and if they followed the DOD internet MIB standard the interfaces should show the correct utilization automatically. So you should be able to add a link from your router to a switch and see the utilization after selecting the up-link interface. If your switches are managed and you can't get the routers to be managed you could use the up-link interface from the switch to determine utilization and just Ping the routers.
If you have to build you own interface counters you will have lots of fun there
search the forum for "Rate".
Lebowski