Currently, it is impossible using Adaxes built-in functionality, however, you can do this with the help of a script. For example, it is possible to create a script that can be executed on a user and that would check whether a user has an Exchange mailbox and would assign the Exchange Online plan only to users who do not have a mailbox.
Then, you can, for example, create a Custom Command that can be executed on users that will activate Office 365 accounts for users using the script. Thus, whenever you need to activate an Office 365 account for a user, you'll need to execute the Custom Command on the user's account, and the script used in the Custom Command will automatically assign the Exchange Online plan based on whether the target user has a mailbox.
If this solution is OK with you, we'll provide you with the necessary script.