There is no simple way to do that with UDD, unless you manually edit the database. Managed Planet has a product called Asset Control that does let you add devices into the manually into the database. It gives you an inventory looking type record for it as well as doing SNMP discovery and interpretation of devices as well. This plugs right into the console and makes your non-managed devices look just like managed computers.
Hey, I wrote a document on how to do this.
Let me know how it works.
If you want to manage other stuff, you can look at the old tool, Asset Manager, or the new Asset Lifecycle Manager.
They are additional things you have to buy, but good tools..