is possible extend a plugin without specific filter or hook?

I have edited and make some custom code in a plugin, now I don’t want to lose the changes when this plugin needs to be updated. I think the correct way is to create a plugin. I’m finding it difficult to create it. because the custom code is in the function in the main class but if I extend the main class and copy the construct is like the plugin is activated the second time.
what is the right thing to do? the plugin not have a hook or filter for what I need.