Is it somehow possible to change MANUALLY the format of a telephone number? In this question Is it somehow possible to change the format of a telephone number being displayed ? @Edelcom asked to change the format of telephone numbers, but he could get his wanted format just by changing his language. By manually I mean that I want to set it up no matter the language such as the format I want isn't in any of the languages I speak.

By the way: my phone is a Samsung Galaxy i5510.
This is a function of your dialer and contact-saving/editing application. Various ROMs use various applications (be it AOSP Dialer, Motoblur, Sense, Touch Wiz, etc.).

You can install third-party dialers. I've never used it but this dialer supposedly supports what you're asking for. I suggest you browse around the Market for third-party dialers.