One way to accomplish this would be to read the value with a DB Input and a custom where clause to lookup "UNKNOWN" and save that in a global variable with tSetGlobalVar. So use: tDBInput->tSetGlobalVar (Replace DB with your database type)
Then you add logic in the tMap if the lookup returned null to replace with the value from the global variable.
ISNULL(row2.MainLookupCustKey) ? (String)globalMap.get("myGlobalVariable") : row2.MainLookupCustKey