Q:

extendable wxwidgets

I was windering if it is possible to create an extendable layout.  This would be: a program or suite where you wouldn't need to change the basic sourcecode to add buttons or text-fields, but rather add them in a file of your own. 

A:

You can build your GUI from XML-Resources:

http://www.blog.pythonlibrary.org/2010/05/11/wxpython-an-introduction-to-xrc/
http://wiki.wxpython.org/XRCTutorial
http://wiki.wxpython.org/UsingXmlResources

You could e.g. use wxGlade to generate these XML files.