Unfortunately, the US IRS doesn't honor retirement savings plans of other countries, but as Fatbrit said, letting it sit where it is without adding anything to it is probably the best approach for the moment. (It's somewhat similar for Americans abroad with IRAs or 401Ks.) I've also found that even "international" tax consultants don't seem to have any idea just how to deal with these retirement savings plans.
Cheers,
Bev