Adding 2 new fields to FA PO Additional Information Window

Client Requirement/Issue/Request. 

I need to add two new fields linked to each other on the FA PO Additional Information Window. Since the FA is a third party, I can't use dexterity code so I need to use the modifier/VBA or Visual Studio tools. Is there anyway this task can be done?

                       

This sounds a small task to add two new fields on window and link them to each other. But as the person who needs that functionality also mentioned that he can't add the field as this is a third party window and was looking for alternate ways like vba or visual studio tools. 

Since he wanted to have them on same window so we decided to use modifier and dexterity combination instead of going for additional menu window and once I provided him solution of his first requirement he come with two more task.

The first requirement was to provide a field on window for sub class and on selecting Class ID the valid sub class will get filled on window. 

 

Second Task:

The second was to link Physical Loc ID field and Structure ID fields. For this we created a custom table to define the relationship b/w these two fields.

Third Task :

The customer has 3 Branches. The branches are included in the chart of accounts which is the first segment. The customer wants when he select the Location ID, the Account group will be automatically selected.