I'm not sure ab out monetdb, but within Sybase ASE you can define views for your tables that convert the Sybase datetime to the format you need. Once the view is created, you can just BCP directly out of the view, and your data should be ready for your import.
You may be able to BCP into a table view, and accomplish the same thing.
More information on views can be found in the Sybase's Transact-SQL User's Guide