There is no "frozen ATPL" for helicopters under Easa.....
I am an EASA examiner (though not in the US), and have done some ST for ATPL's already-its not just a simple check flight like you may think...
First, you need all exams to be valid, and to hold a multi pilot type rating already before you can apply for the Skill Test.
This needs pre approval of your licensing CAA....
This check can be combined with an additional multi engine type rating, but usually can not be combined with an initial type rating on MET.
(Some CAA's don't mind, while others insist on this rule...)..