Yes, when you are scheduling a post, you are presented with a couple of opportunities to save it as a draft.
When posting regular content:
When posting via bulk scheduler:
After you've saved a post as a draft, you can retrieve it by clicking on the Publishing drop down then Post Drafts.
Then locate the post you'd like to edit/publish, and click Continue Editing to bring the Schedule A Post window back up for that post.
You may now modify the post and schedule it when you're done editing: