In Garmin Connect Web, you can import activities in the FIT, GPX, and TCX file formats at Garmin Connect. Also FitBit activity files can be imported there. No idea how you can export activities from Suunto.
However, I would not bother much about importing old activities, and just start from the scratch. Imported activities have no impact on the metrics, fitness level, badges, challenges, or anything else appart of the history, which you can finally certainly see much better on Suunto's own interface.
