Adding custom fields to items in SAP Transaction ME5xN x=1,2,3..

This can be done by following below path −

T-code − SPRO → Materials Management → Purchasing → Purchase Requisition → Define Screen Layout at Document Level and there view the details on the transaction you want to customize.

When you execute, there view the details on the transaction you want to customize. You need to uncheck the "Display" checkbox for your field.