Lansweeper automatically scans network devices if they support one of the protocols mentioned in
this KB article. If they don't support any of these, you could enable "Save Ping IP" under
Configuration\Scanning methods, section
IP Address Range scanning. This will cause Lansweeper to generate asset pages for devices which only responded to ping, but revealed no further data.
Alternatively use the
asset import function in order to import asset data from XLS.