mncomputerguyI would be careful about doing this because I did this in a hybrid configuration for a while and realized it was not the proper way to manage accounts. Now the information in AD is not aligned with what is in the on premise server however the changes are reflected on the Office365 account. Everything still works however if I want to correct this issue I will need to delete the problem account and recreate them in the EAC to get the information consistent in on premise Exchange, AD, and Office365.
Since you don't have an on premise Exchange it may not be a problem for you however it is an unsupported way to manage exchange accounts and who knows what Microsoft may change up in the future. Hope it works out for you though because I am not looking forward to correcting my mistake.
Microsoft Exchange team if you are listening here is a thought:
You currently have Azure AD Connect to sync AD with Office365. Could you have a similar tool to manage exchange accounts and the rest is managed in Office365?
Right now when I add a user I login to the onsite exchange EAC go to Mailboxes>Add Office365 Mailbox. Once I add that the users is added into my AD and then the information is synced to Office365. All other changes I do from Office365 Exchange Admin.
So from my perspective the only piece needed on site is the part to create the user. I am sure there is more to it than that however I hope my point makes sense. Also it is probably easier said than done but I think a tool to bridge that gap would make sense instead of double billing customers.