In reality, you can do anything you want, but, you should really strive to put your data into the standardized forms. This means that each of your ICD9 codes get mapped over to the standard concepts using the CONCEPT_RELATIONSHIP table, using the ‘Maps to’ relationship.
This is a very complicated topic to educate you on a forum thread about how mapping is implemented in your ETL, but you should be able to find threads that point you to tutorial videos on how this is done. There may be other questions on the forums that have this type of questions, you should search for.
In any event, you should not make your own local updates to the vocabulary to make it ‘work for you’: when you have other issues related to vocabulary, no one will be able to help you because your vocabulary is modified and no one will be able to give you an guidance based on your changes. They define standard concepts for very good reasons.