Brix,
The SEP rating used to be included automatically within the ATPL when there were national CAA issued licenses. These days they seem not to be included anymore.
If you don't have a SEP Rating in your ATPL, then you can't move a C172 in Europe either. So you need to get that rating again anyhow if you wish to fly SEP in Europe and following that the US will give you the validation too.