The firmware itself doesn’t directly query Wunderground. Instead it queries server cloud.opensprinkler.com, which runs a nodejs script that further queries Wunderground to obtain weather data. You can definitely host the script on your own server, and then point OpenSprinkler firmware to that new script location using:
http://x.x.x.x/su
where x.x.x.x is your OpenSprinkler’s IP address. There is still a lot of details to sort out here, particularly how you can adapt the script to query your Netatmo station. The nodejs script is available on Github:
https://github.com/OpenSprinkler/OpenSprinkler-Weather
and hopefully it won’t be too difficult to modify it to adapt to your Netatmo station.