A direct vent flue system is in fact one known within the trade as a balanced flue system whereby the fresh air for the burner is drawn down through an outer tube to the combustion chamber and the exhaust gases are vented through an inner flue combined within the whole flue system.
In short the answer to your question is NO – not unless the manufacturer has an upgrade kit available but is not very likely.
Hope this helps.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians