applying function on a value returned from MarkupExtension in xaml

Is there a way to apply a function on a value returned from a MarkupExtenstion such as {Binding ... } or {StaticResource ...} in XAML? Usage example: making the font size of button1 to be twice bigger than the font size of button2?

From stackoverflow

• No. You would have to make a custom markup extension that does this.

  Here's an example that evaluates a Python/Ruby script via a markup extension.