At the moment there is no way for you to do it. Relishapp does the formatting and is not open source.
But you don't need to submit updates in that format. All I do is update the cukes as I normally would, and then push them up to relishapp using the relish gem (which _is_ open source). So if you submit a pull request, I can merge it in and it'll be part of my next push to relishapp.
In terms of formatting, the only place you have any control is in the narrative, and it uses Markdown. Just keep in mind that the left margin is calculated by the position of the left-most characters in the narrative. That's important if you've got any code in the narrative.