Sounds good. Please feel free to reach out if you need any help with incorporating your source vocabulary into OMOP. Especially if you want it to be incorporated as a vocabulary with OMOP concept_ids and so. Vocabulary team has an established process for this.
Re old ATCs: they are not in the mappings we created. So if you need both the concept_ids for ATC to be placed in drug_source_concept_id AND mappings to OMOP you should state so.