As European licences are governed by EASA then the rules are meant to be generally similar across the 32 EASA countries but differences do occur! From a UK perspective, the regulations regarding licence conversion can be found here (although the UK is shortly going to be leaving the jurisdiction of EASA!): https://www.caa.co.uk/Commercial-industry/Pilot-licences/Helicopters/Airline-transport-pilot-licence-for-helicopters/
If you are looking to convert a Canadian ATPL(H) over then you can get credit for your hours / experience in reducing (or eliminating) the amount of theoretica