I have personally tried to get Cisco UCS systems (both standalone and blade chassis) to work with SIM, but they simply do not, at least not as expected.
So, yes there is a way to define new system types. This method works very well for Dell machines and I currently have my SIM instances set up to discover Dell systems successfully which makes auto collection population very nice.
For Cisco UCS, you can import all the MIBs (use the 2.1+ versions as the older ones do not work) successfully, however when you go to modify or view the MIBs event values in SIM you will not be able to see any of the specific events. I believe this is likely the way the MIBs are compiled by SIM. Modifying and manipulating MIBs is a tricky business and I am not knowledgeable enough to do this myself. What you find is that only one of the MIBs will show anything and it is the notification of fault MIB, which has little to no value as the severity codes do not show up correctly.
There is almost no information on how to get this working from HP as they may eventually phase out SIM for OneView (probably a few years out) so do not expect new functionality for other vendors. You likely can't open a support call on this either unless you have a support contract that includes Cisco equipment (we don't).
If you somehow get it working, please post about it. I gave up officially as we have an EMC tool that surprising works well with UCS systems, but we are still figuring out the events/alerts for it.