Remapping Y and Z keys with Karabiner
ahem … Karabiner 10.xx no longer works with MacOS (High) Sierra …

The interim app Karabiner Elements 12.10 will solve your problem very easily, but "Complex Modifications" –meaning modifier/key combinations or e.g. "if_held_down"– can locally only be added/modified by manually editing      ~/.config/karabiner/karabiner.json.

Here's a comprehensive guide for the interested ones: https://pqrs.org/osx/karabiner/json.html