For now the easiest way would be to submit ZARP to workflowhub.eu and then download the packaged RO-crate from there. Then we could add the metadata to the repository itself (so it's a little ping-pong method). However, for that solution we would have to wait until they have a proper stable annotation working already on their side. By "proper" I mean that it takes into consideration all the files of the repository. I personally do not think this will be solved soon.
It might be faster if we write the metadata ourselves. Then again, we would have to crunch the RO-crate specification on our own, whereas the workflowhub people are experts on it.
In my opinion we should wait with this issue and address the others. Workflowhub people are also working on a Python parses that scans a repository and generates the metadata. This seems ideal. Hopefully it will be ready soon and we could just clone it and use on out repository.