Herluf - there is no conversion process. You will have to pass a JAA Class 1 medical, all 14 theoretical knowledge examinations and the ATPL Skill Test on a multi-pilot aeroplane as defined in AMC FCL 1.215. If you do not already have the particular multi-pilot type rating on your non-JAA licence, you will have to complete an approved type rating course before completing the Skill Test.
Babisuri - You cannot take the ATPL Skill Test on a light twin. It must be a multi-pilot type as defined in AMC FCL 1.215. It does not, however, have to be the same type as that on which you built the 500hrs multi-crew time.
Julio - you will have to convince the CAA that your BE200 time was really multi-crew in order to use the time for the 500hr requirement. There are no exemptions and the BE200 is not a multi-crew aeroplane as defined in AMC FCL 1.215. You will, therefore, have to complete an approved type rating course on a multi-crew aeroplane before taking the ATPL Skill Test, after passing the JAA Class 1 medical and the 14 theoretical knowledge examinations.
The chances of a validation, unless you already have the type rating that the sponsoring airline wants, are vanishingly small.
The full list of multi-crew aeroplanes as defined by AMC FCL 1.215 is as follows:
Aerospatiale/Sud Aviation - SN601 Corvette; SE 210/10B3/11/12
Aerospatiale/BAC - Concorde
Aerospatiale/Nord Aviation - Nordatlas 2501; C160 P Transall; 260A/262 A-B-C Nord
Aero Spaceline - Super Guppy
Airbus - A300; A300FFCC; A310; A319/320/321; A330; A340; A300-600ST Beluga
ATR - ATR 42/72
Mitsubishi/Beech/Raytheon - Beech 400/MU300
Boeing - B707/720; B717; B727; B737; B747 ;B757/767; B777
British Aerospace/AVRO - ATP Jetstream 61; AVRO RJ/BAe146; BAC 1-11
Hawker Siddley/BAe/Raytheon - HS125; HS748
Hawker Siddley/BAe - Jetstream 31/32
BAe/AVRO - Jetstream 41
Canadair - CL415; CL600/601; CL600 B19; CL600 2B16
CASA - C212/CN235
Cessna - C500/550/560; C650; C750
Consolidated Vultee Aircraft - CV240/340/440; CV580
Dassault - Falcon 10/100; Falcon 20/200; Falcon 50/900/900EX; Falcon 2000
De Havilland Canada - DHC7; DHC8; DHC8-400
Dornier - DO328-100; DO329-300
McDonnel-Douglas/Boeing - A-26B; DC3; DC4; DC6; DC7DC8; DC9-80/MD88/MD90; DC10; MD11
Embraer - EMB120; EMB135; EMB145
Fokker/Fairchild - F27; F28; F50; F70/100
Grumman Gulfstream - Gulfstream I; Gulfstream II/III; Gulfstream IV; Gulfstream V
Handley Page - Herald
Israeli Aircraft Industries - IAI-1121 Jetcommander/-1123 Commodore Jet/-1124 Westwind; IAI-1126 Astra
Junkers - JU52
Lockheed - L188 Electra; L382 G (C130) Hercules; L1011; L1329 Jetstar
Learjet - Learjet 20/30; Learjet 45;
Learjet 55; Learjet 60
Leteckee - Let L410
MBB - HFB320l; VFW-614
PT Industry - IPTN CN 235-110
Rockwell International - NA-265 Series
Saab - SF340; SAAB 2000
Short Brothers - SD 3-30/60; SC5 Belfast
Vickers-Armstrong - Vanguard; Viscount