No it is not possible to provide access to the Sales Message Builder in the Add-On user accounts. The design is that only one account has the Sales Message Builder and that is the Pro user account. This provides only one account and one place where you sales message can be created and edited.
That sales message or sales messages are then shared with all of the linked Add-On user accounts. This is designed this way for a few reasons. First, this eliminates version control issues where you have different users going in and modifying the message. This also allows management or leadership to standardize the message that all of the sales reps are using with there only being one message in one central place (account).
Lastly, it is probably best to not have sales reps distracted by seeing the Sales Message Builder and spending their time learning about that and trying to create or modify messages. If you do want your sales reps to be the people building and modifying the sales message, share the Pro user login credentials and they can do work on the sales message while logged into the Pro user account.