is it possible to hack a module by mymodule? A few times i had (sadly and not proper) to hack a module, like changing some functions or default images, is it possible to hack a module in a proper way by "mymodule", in order to override simple things like changeing the returns of htlm or css with something like

mymodulefunction_moduleoriginalfunction(%args)


and make sure "mymodule" overrides my code running after original module?
No, it's not possible in the manner you are describing. However, Drupal provides many _alter() hooks that allow various things to be altered as necessary, so that you don't have to hack anything.

However, without specifics on what you want to do, I cannot suggest any specific methods.