A Part-FCL ATPL, which is what I assume you mean by a 'UK EASA ATPL', cannot include any non-EASA rating/certificate such as a Microlight Class Rating.
Simplest way is to revalidate your SEP Class Rating, then do your Microlight conversion. Bear in mind that, under current rules, Microlight flying will not maintain the validity of your SEP Class Rating, so at the very least you would need to fly a revalidation proficiency check when your SEP Class Rating nears its validity expiry.