SPM,
For starters, I assume your ATPL is for Airplanes and not Helicopters, the regulatory situation differs between the two.
There is no conversion arrangement in JAR-FCL as of now (Except for PPL). However, JAA has adopted a crediting-system, i.e. you are given credit towards the training requirements relative to your experience level. Minimum requirement is you'll have to sit all exams and do a Skill Test, though. All details are
here , and you find it in Subpart A, page 1-A-4, item 1.016 - "Credit given to a holder of a licence issued by a non-JAA State".
Spain has adopted JAR-FCL and should consequently have ceased to issue 'national' licences.
cheers,