Of course! When scheduling a post, you will be presented with a couple of opportunities in which you can 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 Publishing > Post Drafts.
Then locate the post you'd like to edit/publish, and click Continue Editing to bring the Composer Window back up for that post.
You may now modify the post and schedule it when you're done editing: