Is it possible to define a custom numbering style in Pages? I would like to cite some articles, books. The standard form is to use square brackets, but Pages these are not within Pages numbering styles. I want to achieve: 

[1] 1st source 

[2] 2nd source

.

.

.

[n] n source

Is it possible to create a completely new style? I know that I can redefined style in the terms of spacing, but I have not find an option to define own "bullet buttons".
Unfortunately not.
While you can indeed define custom bullet styles, you cannot modify the predefined numbering styles or add a new numbering style.

You can use Apple's feedback form to send a feature request.

https://www.apple.com/feedback/pages.html