Right now, the tool I have for modifying ROM3 isn't very flexible. I wouldn't even call it an editor since all it can really do is inject data for an AID from a template and recalculate pointers. In the long run I'd like to create a more generic version where you can customize the bitflags/labels to inject any sort of AID without the need for a template. In theory, that would allow you to inject any kind of block you want into any of Fates' asset files.
I'm not as familiar with the files used for custom DLC, but a good portion of the tools should be compatible right from the start. If not, the editor relies entirely on pointers rather than hardcoded offsets for locating tables, so configuring it to work with custom DLC files shouldn't be too difficult. At the least, the injector for ROM3 should work with AID files for custom DLC since it just adds the AID to the end of the data region.
I'm not as familiar with the files used for custom DLC, but a good portion of the tools should be compatible right from the start. If not, the editor relies entirely on pointers rather than hardcoded offsets for locating tables, so configuring it to work with custom DLC files shouldn't be too difficult. At the least, the injector for ROM3 should work with AID files for custom DLC since it just adds the AID to the end of the data region.