I found how to do it.
Firstly, I replaced all *.tres by equivalent *.gd.
Secondary, I replaced all export-variables by consts.
After that I able to do this:
godot::Ref<godot::Script> scene = (godot::Ref<godot::Script>) godot::ResourceLoader::get_singleton()->load(paramsResourcePath);
godot::Dictionary params = scene->get_script_constant_map();
godot::Godot::print(params["DefaultRounds"]);
Now I can use *.gd scripts for parameters that can be changed without game recompiling.