3812878 - What do I need to consider before requesting to add a field to the expense entry form?

Our Dubai/Bahrain office would like for the 'Merchant VAT number' to be entered by the employee on an expense claim level (by line). 
Please can you advise where this can be found and if this can be added to each line detail so that the employee can enter the necessary details?

Client wants to add a field to all expense entry forms used by specific users , KBA , CON-EXP-ADM-FOF , Forms and Fields , Problem