1. it's possible that you don't have permission to use that field -- not sure which groups can add related tasks. If you can't do it, just leave a comment. Someone with the appropriate permissions then can add the relationship.
2. I don't think we have a completely clear rule on this. I'd personally prefer to add a new patches-Task, make the original task a related one and then close it pointing to the new task. But IMO it's also ok to simply add it as a comment to the original task. Someone with the appropriate permissions can then move the original task to patches category.