Do it the right way around - or it may not be possible to do it at all!
When applying for a new Rating you now have to apply for your paper EASA
license at the same time.
As far as I am aware it is NOT currently possible to add an IMC / IR(R) to an existing EASA License - it can only be "Grandfathered" if you hold it on a UK Issued JAR or UK License.
Therefore do your IMC Rating FiRST and your new EASA License will have it (IR(R)) on it - "Granfathered" from the date of your test.
The possibility of adding new IR(R)s to an existing EASA License is still under
discussion - but EASA are not keen on National Ratings.