IO,
I'm no expert in conversion for experienced pro pilots, see LASORS for detail. IIRC, there is no way to avoid the full set of JAA ATPL exams in order to get a full JAA ATPL.
The rest is down to hours. Worst case, a CPL skills test (training as reqd) and an IR skills test (15hrs min). Under EASA FCL, the LJ45 TR can be carried over. More typical/better JAA case, a TR checkride on the LJ45 and that's it.
brgds
421C