I queried this when FCL.945 was first introduced and the answer was an emphatic 'NO'. If you are an FI with FCL.945 privileges, you may only sign off applicants who hold a licence issued by the same MS as the one which endorsed your FI privleges.
Even an FE may only revalidate holders of licences issued by other EASA MS by Proficiency Check, not 'by exeperience' and only then if the NAA of the applicant allows this in the Examiner Difference Document.