use Watchlists for unknowns
If we manually change the unknown devices to a known one something like the Unix Flavoured devices(AIX,Solaris, Linux), the concept of auto discovery fails and new devices coming in will be unknown as well.
What i have done is, in our environment we were having 900 Unix flavoured devices, I made a common device XML named "Unix_Flavours.xml" and copying all the XML Messages from AIX,Solaris, Linux XMLs and removed the Unix AIX, Unix Solaris and Linux device XMLs.
Now all the Devices are getting discovered as Unix_Flavours which are reportable.
Note: You should be knowing UDS Programming to do this. This is not the proper way but the best option possible with RSA enVision
if you have any questions please let me know.