The answer, as always, lies in JAR-FCL 1, in this case Appendix 1 to JAR-FCL1.205 - The holder of a single-engine IR(A) who also holds a multi-engine type or class rating wishing to obtain a multi-engine IR(A) for the first time shall satisfactorily complete a course at an approved FTO/TRTO comprising at least five hours instruction in instrument flying in multi-engine aeroplanes, of which 3 hours may be in a flight simulator or FNPT II.
You will then, of course, have to pass the IR Skill Test in a ME aeroplane.