Here is a tutorial I wrote about modding the co-op quest mode in Towerfall.
A comprehensive (I believe) list of monster and item names to use in XMLs is also included.

https://mega.nz/#!f8sURRbQ!P_WQRGO_tPkPe6t_izio368qCnJqrEoWDFPOB4WLaQQ

Don't be scared by the length of this tutorial - once you get the hang of it, it takes 10-15 minutes to create a quest mode map!

Oh, and I was wrong - you actually can add maps to quest mode without replacing the originals. I might update this tutorial if I have some time later.

Edit: Updated the tutorial.