Whether or not you can pass the exams without approved training (and the concensus appears to be not) is irrelevant unless you qualify for exemption. To qualify, you must meet all of the experience requirements for the issue of a JAA ATPL(A), you must also have a minimum of 1500 hours as pilot (PIC or Co-pilot) on multi-pilot aeroplanes (or single-pilot aeroplanes operated by two pilots according to operational requirements) and hold a valid multi-pilot type rating for the aeroplane to be used for the ATPL(A) Skill Test.
If you do not meet the experience requirements for the issue of a JAR ATPL(A) (Including the 1500 hours MPA) and wish to take the ATPL(A) theoretical knowledge examinations, you will have to first complete the full 650 hours of approved training
LASORS D1.5 and G1.5 refer.