localhost Flarum supports Markdown and bbcode syntax for image embedding.
If you want to import attachments, you could migrate them to FoF Upload, but there's no built-in import tool.
If you want YouTube URLs to turn into embedded players, see FoF Formatting and its MediaEmbed option.
EDIT: and if by copy you mean copy from a database to another directly, you'll need to parse Flarum posts with s9e/TextFormatter before storing the parsed version inside of the Flarum database.