I have just seen in this post, which is kind of related with my question, how to create a property with code. But I would like to know if it is possible to add a custom variable to the property tree using any of the ".xml" files of FG. I just want to add a simple variable, lets say an integer.
In here it says "In addition, it is made very straightforward to publish/expose new variables from C++ space to the property tree. In addition, new property tree variables can be easily created using XML files or Nasal scripts". Any idea?