Is there an importer than can read from text files, like csv files ?
If so this seem to be the simplest approach for importing data from other sources because I can extract data from the old forum without problem, the difficultly is getting an import into discourse.
There are a couple importers that use csv. It’s usually better to use a database though.
You can use the API, but it’ll be much slower. If you have only a few thousands of messages, it could be a solution, but it’ll be easier with an import script.