export R neuralnet package model to PMML
You already mentioned the alternatives...


convert the neuralnet object to a nnet class object
implement pmml.neuralnet()


Both pmml, nnet and neuralnet packages are easy to read. You can find the source code over cran.

I think implementing pmml.neuralnet() might be the cleanest. You can just copy-paste the current pmml.nnet()  method, call it pmml.neturalnet() and make the necessary changes. Should not take more than a day's work.

ps: It would be cool if you'd sent your function to neuralnet authors afterwards of course, and link it from here. :)