Since OpenSprinkler uses WUnderground.com to obtain weather data, only weather stations that are registered with WUnderground may be used.
However, since the firmware makes weather query through a script, it is possible to set up your own script that makes use of your local weather sensor data. Alternatively, if you have a server that queries your sensors and calculates a watering level, you can have the server call the existing OpenSprinkler API (i.e. setting the ‘wl’ variable) to feed the result to the firmware. This way, you can integrate data from your own sensors and not be restricted by what the default script provides.