That sounds more like a feature to be added to FitNesse rather than something you can do in your fixture currently. There is no mechanism in the code for this right now.
Feel free to post a issue to https://github.com/unclebob/fitnesse and you are welcome to contribute a solution. Now the solution will probably need to be done two ways, as FitSharp doing FitLibrary would need to work one way and the Slim implementation would be different.
I like the idea.