Actually I simply replaced the idle_breathe-animation with a custom one and did not try your approach to add it yet.
For sure I will share how to do it. However, to follow the description you will need the Add-On I am working on for Blender - or a similar one for another tool. If you do not have it you would need to read all the information from your animation (rotation, location, etc.) and write it manually into a file. So as long as the Add-On is not in a better shape it might not make sense to go too much into details… the progress can be followed here.