That's indeed something that's not directly supported by a QuerySpec construct, so you have to build it using the low level API, as in, create a FieldCompareSetPredicate, where you pass in PersonFields.PersonId as the field and create a new field with queryFactory.Field() and set its Expression property to getSubordinatePersonIds, and use In as the Setoperator.
so something like:
predicateExpression.AddWithOr(new FieldCompareSetPredicate(PersonFields.PersonId, null,
qf.Field("personIds").SetExpression(getSubordinatePersonIds), null, SetOperator.In, null));