You'll want to build an exporter that will work with nodebb-plugin-import: https://github.com/akhoury/nodebb-plugin-import/blob/master/write-my-own-exporter.md

It depends what forum you were using before... we have specific exporters for most of the major forum software platforms. If it's custom then you're looking at having to write your own or contact us at [email protected] for a quote.