Check out a web app called QBox, it's primary purpose is to benchmark your training data, but it also lets you benechmark against multiple providers. If you run a test by uploading your DialogFlow training data, and select IBM Watson as output, you'll get an option on the resut page to download the training data in IBM Watson format.
Select Watson after you've uploaded your training data:
Run the test, wait a few minutes, and on the results page use the menu on the top right to get your Watson formatted training data:

(Disclaimer: this is a tool I work on, so I have not provided a link, but if you Google the name 'Qbox' along with the term 'chatbot', you should find it!)