If i have AlphaBunde
that is parent of BetaBundle
how can i extend a twig block without overriding the entire template?
How can i import the routes that exist only in BetaBundle
?
You cannot extend a block in Twig. You can overload it if you extend a template (and call parent() which kind of works as inheritance).
If you extend bundle you can overload its controllers or resources. I don't think you can really extend a template from a parent bundle because of how the paths are resolved. You can read more about it in the Extending a Bundle documentation chapter.
Also, How to use Bundle Inheritance to Override parts of a Bundle might clarify few things.
Remember to check Overriding Bundle Templates to learn how to overload templates in an application.
About routes: I think you'll have to define each route in your application's configuration file if you want to import them selectively.