I went the other way from ATPL(H) to ATPL(A) and the conversion consisted of PofF, Performance and a 100 question bridging exam covering the specific differences between the fixed and rotary exams. That was with 4500 hours rotary twin turbine time though!
More interesting though is why would you do it? Helicopter licences are very, very expensive in order to gain relevant empployable skills but paid far less than the fixed wing counterpart?
Great fun to fly but they don't pay the bills these days.