Hello Abdel, what are the customizations you wish to do?. If you need to declare some new custom field on PO form and report, you can simply do it from Frontend by enabling developer mode. By enabling the developer mode, you can see the "Technical " Menu under the Settings Menu. Under the Technical menu, you can see all the technical menus mainly for the User Interface, Data Structures, Actions, Security,Reporting, etc. By this approach you can simply make the changes as per your need.

Or you can develop the backend module by inheriting the core purchase flow..