How do I scan multiple subnets?
1 Spice up
If you go to http://spiceworkshost /settings/network you can click the button that says Add IP Range. You could also install an instance of Spiceworks on the other subnets and have that instance report back to the main install.
http://community.spiceworks.com/help/Remote_Collectors
2 Spice ups
williamnb
(WillNB)
3
Go to Settings > Network Scan and click the Add IP Range button. Easy! 
williamnb
(WillNB)
4
Aah, beaten to 1st answer because I answered an email 
Oh well, as long as you get it sorted.