@Diego Mirner I was researching on this, though i dont know the actual need for this requirement, however you can create a new sync rule on the Azure AD Connect/sync server, where you can create the rule to sync the attribute from on premise AD to another attribute in Azure AD ( which ever you want) this is much better approach to have the attribute information stored in two different attributes within Azure AD.
Let me know if you have any further questions, feel free to post back.