How do I define a SQLite expression in DynamoDB 'object-mapping' DMS migration task?

I am migrating data from a MySQL DB to DynamoDB. I am using object-mapping rule to transform the data to match my needs. However I have a EPOCH timestamp stored in MySQL DB that I would like to transform to Datetime before loading into my DynamoDB. I see there is a transformation rule I could use to by leveraging SQLite expressions. However I couldnt use transformation rules in conjunction with object-mapping. Is there any other way I could transform my date field by still using 'object-mapping' rule ?