OBB files are just opaque binary blobs. Meaning, any file with a .obb extension is valid. I've only had to use OBB files on native projects with big video files in the past. For those, it was just a matter of changing the .mp4 extension to .obb, and everything kind of works out of the box.
The version of android that's found on phones supports two OBB files, up to 2GB each, totaling 4GB. This is different from what we support. I wouldn't be surprised if tools like aapt had a hard coded limit at 2GB. Unfortunately, working around this limit is not going to be easy.
What i would do is package additional content in unity asset bundles. Put all the asset bundles in a zip file (Which has a convenient limit of 4GB anyway), then change the extension of the file from .zip to .obb. You then need to write some application logic to either stream asset bundles out of the obb file and load them into unity, or to unzip the obb file, then just load assets off the disk. I suggest streaming over unzipping to save disk space.
From what i understand this is the best way to make things work. I could be wrong and there might be a cleaner solution.