Sonos doesn’t really provide a way to ‘merge’ systems. You can add the other speakers to your current system, but I don’t think there’s a way to merge playlists, etc. 

Adding speakers to another system means resetting the speakers you'd like to ad first, so you'd lose all playlist an presets on them. This could be a problem depending on how you've set up the system.

Also what does “two different networks” actually mean? If its two subnets then you can’t merge them: every Sonos device in a Household has to be on the same subnet.