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 theoretical knowledge training and flight training required before sitting the written exams / skills test. However you aren't exempt from the exams themselves so you still have to pass the 14 written exams and the skills test. If you don't meet the ATPL(H) requirements then you'd be looking at getting a CPL(H) + IR with the 'upgrade' to an ATPL(H) being done once you meet the hours requirements.
Currently, issuance of a UK (EASA) ATPL(H) based on a ICAO compliant licence requires:
- Class 1 Medical Certificate (has to be issued by the European country which will subsequently issue your licence)
- a pass in the 14 written exams
- minimum flight experience (e.g. 350hr multi-pilot, 250hr PIC, 200hr XC, 100hr night, etc.)
- pass a skills test
The other thing to watch is that you have to have a valid type rating (endorsement) on your licence for the same type of helicopter that will be used for the skills test. Some military helicopters may not have an associated civilian equivalent so you'd need to cross reference with the EASA list (https://www.easa.europa.eu/sites/default/files/dfu/15102020 EASA T_R_List_Helicopter.pdf).
If you decide to sit the written exams then feel free to send me a PM as I'm going through the process at the moment!