Appending trace to waterfall plot

Hi!

I have a question regarding the waterfall plot. I would like to append a trace to a waterfall plot using its right (the inclined) axis. Is this possible and if yes how should I do it?
Thanks for any ideas and HOW-TOs!

Cheers,
Gregor
A waterfall plot displays the columns of a single matrix so the only way to add a trace is to add a column to the matrix.