I would like to modify Pages a lot. For example, I would like to create a galaxy Page with rss-reader written in PHP (or javascript) in it.
I found where content is stored for Pages, but not the script who read this content.
Pages uses the WYMEditor to render Pages:
See the wym*.[css/mako/html] and jquery.wymeditor.js files for details.
Is there any possibility to integrate code into Galaxy Page which could be executed ?
It's not clear exactly what you're looking to do, but Pages includes JS code to load histories/datasets/workflows on expansion. You might look there as a starting point for executing code in Pages.
Let us know if you have more questions.
J.