How to import custom comments of the translator during TMcleanup ?

We have several custom fields which can be filled with static values during cleanup. But there is another field containing comments of the translator filled in during translation. Any suggestion on how to import these  values in the master TM as well?
So far we tried to do a normal TM cleanup followed by a TMX Export and then updating the master TM with the TMX. Is this the only way to retrieve the "custom field values" or can we make use of the API during cleanup to acchieve this?