I've created a SF DevExpress Maintenance form and I want to know if i could add my own buttons or what not to the MaintenanceFormToolbar.

You would have to do this at run-time.  The MaintenanceFormToolstrip is nothing more than a pre-built toolstrip control with some built-in functionality.  You can reference the Items collection and do anything you want.  ANything you do at design-time is going to be over-written due to the nature of the collection and how it is managed through .NET.