The idea of "first drafts" and "second drafts" is a concept from our school years, from when we sometimes had to turn these things in labeled as such.
It also comes from the way most writers used to work...handwriting everything then typing out a draft (or paying someone to type a draft). Or it could refer to the freshly typed version a writer made, after marking up the old one, for those writers that mostly wrote directly on a typewriter.
Even then, some writers would go back and edit individual chapters (or other divisions) before moving on to write more prose. While others would plunge ahead and not touch the first chapters until everything was done.
Now that people generally write directly on computers (or at least that's where the typewriting happens), the concept of a draft isn't as clear. What used to be a few passes with the red pencil (or another tool of choice) before creating a new draft now gives clean copy with every touch of a key.
Wherever you are in the writing/editing process, if it works for you, it's valid. If a teacher or publisher or editor requires a new draft from you, you can create a proofread copy and call it "the second draft" or whatever is needed. If your drafts are just for you, then they can be whatever you need.
So which answer are you hoping to hear?
Finish that story! Don't rewrite your earlier chapters until you know how you're going to write that plot ending. You're just putting it off because it's hard.
If "fixing" the story from the beginning is going to help you write that last chapter(s), then do it. There's no need to do things in a prescribed order.
While I am definitely in the second camp, I'm not you. Maybe what is in your best interest is to force yourself to write something, even if it's crap, and get it "on paper." Then go back and fix it all.
Or maybe what you need is permission to let go of the idea of a first draft and what it entails. You could commit the ending to an outline on paper (computer) or you could just start at the beginning and deal with that. Rewriting is still hard work and the ending will come when its ready.
The answer to your question is, yes, you can. But which path you take depends more on your process and emotional needs than an definition of "draft."