[Damo]

In Australia we have different fittings for nitro and co2 bottles, not interchangable.
I dont know if theres an adaptor but you could maybe swap the fitting and stem on the regulator to fit the bottle you want to use, take your reg down to BOC or somewhere like that and tell them what you want to do.
Another issue you may run into though is BOC will most likely tell you that the seals in nitro and co2 regs are different and may fail...
What brand of reg have you got?

 

[Damo]

CIG is pretty common, your reg probably has a type 30 fitting, standard for co2 in Australia, what you will need is a type 50, standard for nitro.