The ATPL has no inbuilt IMC privileges.
In order to add an IMC, or IR(R) to an ATPL you simply complete the IMC skills test, you receive a credit against all training and ground exams.
If you hold a Multi Pilot IR, this offers zero privileges when operating as single pilot.
It's worth bearing in mind however, you historically could apply for an IMC rating at the same time as your singe pilot IR - the skills test, and exams covers both. It used to be just a couple of extra £'s. I believe it's the same under EASA