Is it possible to discover non-Cisco devices with the LMS 3.1 and have it displayed (as unreachable) in the topology view ?
You can manually seed (as opposed to automatically discover) a non-Cisco Device into the Device and Credentials Repository (DCR - in Common Services), but it won't give you any real functionality in the topology view (part of Campus Manager) or any other CiscoWorks applications.
As it is not participating in Cisco Discovery Protocol nor reporting it's ports connections via a CAM table and arp cache dump etc., it would be at best an unconnected and unfunctional icon. You cannot manually connect it or otherwise add functionality to it from either the CW GUI or command line.
About all CiscoWorks is good for with respect to non-Cisco devices is as a generic syslog collector. Even there, the functionality is minimal and easily matched by free or open source tools such as Kiwi syslog reporter.
Hope this helps.