Slightly unclear, this one.
Nothing in the ANO Schedule B says that you are required to undergo any particular certification before you can use your IMC rating in a multi-engined aircraft.
LASORS, which is not the law, says that you should undergo certain specific training on twins before being tested for an IMC rating in a twin.
Where an IMC rating is already held, having been tested in a SEP there appears to be nothing in law to prevent you from exercising those privileges in an MEP.
Should you though? ..... IMHO, absolutely not. Get some training.