Yeah! You can enter, there's just a button which you click to show you've made assets before the jam time started.
Traditionally in jams you complete them (or don't finish completing them sometimes, as I've done in the past) in the jam time. Some jams last a week, some a month, many a weekend.
I personally think storyboarding is fine, like taking code from games you've made in the past, but making assets beforehand should be marked.