Has anyone used and can recommend an on-line flight school in order to convert my FAA ATP to a JAA ATPL?

Your FAA certificates don't readily convert into JAA certificates. You will have to sign up for an ATPL theory course before you can take your tests (I think there are 14 tests in all). There is an exception that allows you to directly take the tests, to be eligible you are required to have many flight hours in a multi crew type rated airplane.
Once you have done your ATPL tests, you need to at least take a check ride for instrument flight. The check ride is done in a JAA country. There may be other flights involved in able to satisfy the JAA of your level of qualifications.

As for the on-line schools, you are still required to attend a few weeks of in house training for the test prep.

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The easiest and fastest way possible to have a JAA certificate is to obtain a "temporary" one year certificate through a contract a company that hires you and sends you to formal school in the US that has an approved JAA training program. That training only lasts about a week.

A former colleague, already Lear 45 rated, did just that. He attended the school in Dallas, passed the check ride and was flown on British Airways to Europe. He worked six weeks on then six weeks off until the termination of his contract.