In order to unfreeze your ATPL you need to get a type rating on an aircraft that was originaly certified multi-crew. However the hours spent multi-crew in an aircraft originaly certified single crew may count towards the 500hr multi-crew requirement so long as it is in acordance with company sop's and you get a company letter to that effect.
Pilots of A/C such as the 406 king air etc will still have to get that type rating (and new job, bond, loan etc etc) before they can unfreeze the ATPL.
Hope this helps