What Alan said :-)
A hook for a user-supplied "conversion" script/program had always been on the "to do" list but was never implemented: A lot of people might be happy writing a python script to read standard input, convert, and write to standard output but might want to use kplex to do all the messy networky stuff.
To augment Alan's excellent suggestions you can use kplex to do both the reading (from serial, network, whatever) and writing with your script in the middle of a pipeline. e.g. if you're reading your old wind data from /dev/ttyUSB0 at 4800baud:
kplex -f- serial:device=/dev/ttyUSB0,direction=in file:direction=out | YourProgram | kplex file:direction=in
...where "-f-" tells the first kplex instance not to use a configuration file and only use command line interfaces which in this case is a serial input and standard out as output ("file:" with no "filename" defaults to stdin/stadout).
For added complexity but greater neatness you can do it all from within one kplex instance:
* Create 2 FIFOs in the filesystem (with mkfifo), one input one output
* Have your kplex config output only the old style wind sentences to the output fifo
* Have your kplex config input form the second fifo
* filter all other outputs to discard the old style wind data
* Have your program/script read old style data from kplex's output FIFO, convert, and write back to kplex's input FIFO
...more detail on request but this may be more complexity than you're looking for