Hi,
I've not seen anything that will do that in one go, though I note on the Cumulus forums
this thread is about an application from the Cumulus author that will convert a wlk file to a Cumulus monthly log file. Once the weewx utility
wee_import is released
wee_import will be able to batch import the Cumulus monthly log files (assuming the Cumulus app does its job - I have not used it).
Gary