RFBIBL01 : Customizing poosibilities via User-Exit?

Hello everyone,

I am in need of using the program RFBIBL01 but I need to add a customization to it. Indeed, I need to add a section where I validate some datas in some specific ways before continuing the process. Though, I was looking to add an user-exit and unless I misunderstood the concept, I can't seem to find any. Is there a way and a place in that program where I could make som validations about some datas coming from BBSEG and possibly some others?

I would like to know where to add an user-exit in this program and I can't find it.

Thank you for your kind help in advance.