Function Module Enhancements

Function Module Enhancements are used to extend the functionality of standard SAP function modules. Only certain function modules can be enhanced. Additionally any function module that is part of the Central Basis can not be enhanced (for example: function module ‘POPUP_TO_CONFIRM’).

Just open the FM and select Enhance Interface

Enhance Interface in Function Module
Enhance Interface in Function Module

Any new Import,Export,Changing,Tables parameters can be added now.
Function module Parameters Enhancement
Function module Parameters Enhancement
For FM having enhancement option there may be special tables reserved for enhancements.
Special Function Parameters reserved for enhancements
Special Function Parameters reserved for enhancements

Ex: EXTENSION_IN and EXTENSION_OUT . Go through the documentation of these (click green light button/long text) and modify accordingly.

For function module source code enhancements , its similar to any SAP program enhancements(Implicit and Explicit Enhancements) . Click click on   and see all enhancements available.

Implicit Enhancements In Function Modules
Implicit Enhancements In Function Modules

Read more about Enhancement Frameworks to learn about source code enhancements.