It is possible to write a macro and register it to run after an entity is added?
This is not possible now. You can add attributes to existing entities, not to newly created on some “OnEntityCreate” event. Nothing like that exists in TDM.
What can help is explained here:
Alternatively, it is possible to use Gallery. You can create entity with predefined attributes and add the entity to Gallery (or just attributes). Then you can use Copy & Paste to add the new entity to your model. This is a workaround.