Hi! I am pretty sure you actually can use an already posted story, you just create a new series after April 30 starts and post it there, so I'm pretty sure pre-existing characters are fine as well. Some staff (?) explain it in a Webtoon contest youtube video.
Here is some information on their notice page:
7. Can I submit a series that has been previously published?
Yes, but for this contest, we highly encourage new original content that has never been published. If you would like to submit a series that was previously published online.
- It must have been self-published.
- You must fully own the series and have exclusive and direct control over it.
- It must not have been published by a publisher.
- It must not have been printed offline for circulation including self-published stories.
Hope that helps.