Hi,

 One option is you can keep a boolean field, and if submitted you can check that field by edit. Then  by overriding the write() to check if state = draft && boolean field = checked, do transition to "submitted" state. In submitted state you can make visible the default buttons on open state.

Another option is you can have a button "Submitted", which on click, makes the transition from "draft" to "submitted" and also check the boolean field. So when the boolean field got checked, then only other default buttons in open state would be visible.

You may refer this post to understand, how to change existing workflow by including a new state:

https://www.odoo.com/forum/help-1/question/how-do-i-add-new-state-and-change-the-workflow-of-purchase-order-5118 

If you have any doubts, I can help you with the code.