How to add new fields in standard table ?

Create Z-Structure with the fields needed and append the structure to the standard table by using the option append structure within table display

we can enhance the standard tables by using Append structures and customizing includes.

Append structures allow you to attach fields to a table without actually having to modify the table itself. Table enhancements using append structures therefore do not have to be planned by SAP developers. An append structure can only belong to exactly one table.
In contrast, CI_includes allow you to use the same structure in multiple tables. The include statement must already exist in the SAP table or structure. Table enhancements using CI_includes do, however, have to be planned by SAP developers