For Windows computers specifically, you could use our LsPush scanning agent, as mentioned in Jeremy's post as well. LsPush is a small executable that, when run on a Windows computer, scans the computer locally. You could deploy LsPush with the /folder parameter in the secondary networks, through a logon script or group policy. The scan results will then be placed in the folder you've specified. From there you can copy them and place them in the Program Files (x86)\Lansweeper\Service\import folder of your main Lansweeper installation, where they'll be imported. More info on LsPush can be found here.
If you want to inventory non-Windows machines as well, you would need to set up some kind of connection between your various sites or make your SQL Server accessible over the Internet. Theoretically, you could set up a separate Lansweeper installation (database, scanning service, web console) in each site as well and merge the database afterwards. You would have to write your own database scripts for this though. This is not something we can provide instructions or support for.
