Basically all that is needed is to get weewx to create a log file in the same format as one of the existing supported programs, Weather Display CumulusMX etc and then point the collection program (WXSimate) at that set of files and let it do the rest. Personally I'd pick CumulusMX as the monthly log file from that seem to be all that WXSimate uses. I would not try to copy the format created by WXSimate as that could and probably will change over time.
Stuart