Spybus
To convert a fixed wing IR to helicopter (in Aus), you need to complete 20 hours of dual instrument instruction in the category of aircraft - in this case a helicopter (category meaning either helicopter of fixed wing). Will obviously have had to have passed the IR theory exams which I assume you would have done already.
The training must be done with a flying school APPROVED to give instrument flying training.
The flight test must be conducted on an instrument rated helicopter. If you do the test on an IFR single, you get issued a single engine instrument rating. What can you do with this rating??? Not much.
If you have the bucks, go the multi-engine route.
Good luck
