HeavyWeather
La Crosse weather stations
The software shipped with the device is HeavyWeather, and while it peforms its role of reading, displaying, and storing data from the weather station, it is quite limiting and inflexible. Of course, if you are reading this you have probably already switched to Cumulus and found its advantages.
Using the Heavy Weather program, you can open the History window and save the weather data from the desired history file as a text file ready to import any pre-Cumulus observations. See Monthly_log_file or EasyWeather_Format for more inspirational tips on such imports.