Take a look at this which explains that your saved networks are all in
/etc/NetworkManager/system-connections/connectionNames
Take a copy and restore by putting them back there. Network Manager should pick them up when pasted back into that folder.
You may need to go into the individual connections (in the NM applet) and make all passwords public (unencrypted) before doing the backup, otherwise the encrypted ones will be stored in keyring or wallet, and not in the above clear text files.