The CGA 326 fitting found on a nitrous tank is different and would either require re-valving or an adapter to the CGA 320 that is on CO2. I am not sure they make an adapter for those two fittings as it would highly abnormal to adapt those CGA's for any reason. Re-valving isn't cheap and would end up costing you more unless you know someone in the business, otherwise, once re-valved you could use it. CO2 can be placed in almost any tank, and is often placed in the older tanks due to its lower pressures.
I am sorry, there is a fitting adapter to go from a CGA 326 to CGA 320 for $22. The issue is no supplier will fill it as their filling connections are CGA 320 and they will not adapt for filling for safety reasons...... unless you find some tiny hole in the wall place that seriously doesn't care.