Create a field (defined within same module) during module installation?
The function hook_update_N() only gets called by running update.php. You also need to implement a hook_install(), for example like this:

function mymybase_install() {
  mymybase_update_7200();
}


Do note that this only runs when you enable the module for the first time. So if you already enabled the module, you can test this by first disabling it and then uninstalling it. If you want to run the function each time the module is enabled, you should use hook_enable().