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:

Save_as_draft_compose.png

When posting via bulk scheduler:

bulk_scheduler_save_draft.png

 

After you've saved a post as a draft, you can retrieve it by clicking on the Publishing drop down then Post Drafts.

Post_drafts.png

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.

Screenshot_2017-10-02_20.57.28.png

You may now modify the post and schedule it when you're done editing:

Screenshot_2017-10-02_20.56.40.png