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

Compose Message

When posting via bulk scheduler:

Bulk Scheduler

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

Select 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.

Edit post drafts

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