Can we update standard plugin as a custom plugin with some improvements?

We are working with plugins and need solution related to that. We have a standard plugin (let's say Debrief). We downloaded that, made some changes in it and uploaded it. It is not working when checked. Now we want to know if:

1. we can upload standard plugin as custom plugin by doing some modifications in it.
2. How can we upload it? I mean a custom plugin works well if we upload a zip of JS, CSS and HTML. But, standard plugin like Debrief has lot of files. So, how can we upload it?