Unless you have thousands of hours via an ICAO commercial licence, you would have to enrol, and pay for, a full ATPL course under JAA rules. There are certain stipulations in that there is a minimum number of groundschool self-study and class room hours required.
If however, you wish to start some self-study with some second hand manuals whilst you are hour-building (say), then there's no reason why not but you would still have to pay for a full course.
You may want to check with a course provider, but quite often the fee payable is valid for a few years so you could enrol now and start studying. However, you have to bear in mind that you can only attend 6 exam sittings which means that you can't just do 2 or 3 exams at a time.
Cheers
Whirls