The Nomin Mapper instance is available in mappings. You can access it using its name mapper as follows.
mappingFor a: Person, b: Employee a.children[0] = { mapper.map(new ArrayList(b.details.kids)[0], Child) } // using in the expression a.children[1] = mapper.map(new Kid("The Second"), Child) // using once converted the Child instance, don't know why :)