Very low interest in this I'm afraid, thus whilst it is on the feature request list, its priority isn't high.
If it were to be implemented, as it's pretty much a one shot job, I'd probably do it in something like Python wrapped in a an app as a standalone tool. It isn't that difficult, just read each line of the export and generate the line of data WeatherCat would want and append it to each monthly file, but as I said, the priority isn't there at the moment.