AFAIK this is not a thing in classic dwarf fortress. I was wondering if this can be done now, and if not, (not sure if feature requests here are monitored) this is something sorely missing particularly now that we have steam workshop.

If I need to redefine an entire creature just to add e.g. a new caste, or add/replace a particular tag, that makes my mod (no matter how tiny it is), completely incompatible with every other mod for that creature. Real world example: I want to see if I can add/replace gem shapes (for flavor purposes), but to do that I need to replace the entity raws so civilizations can use any additional gem shapes. This is prohibitively heavy for a mod that doesn't even make any changes to entities.

Is there a way to handle this gracefully?

What I'd like to be able to do is flag that if my definition collides with another definition, its contents should be appended to the definition that already exists. Now that we have mod load order, this should be possible? I also want to be able to modify particular tags on an existing definition, so maybe a way to achieve this would be to specify a specific tag that matches 1:1 in the original raw to delete, which would include deleting any sub-tags that applied to that "root" tag if applicable.