I am not on the staff, but the rules regarding this seems clear enough.
Tapas is specifically looking for fresh material. I was planning at one point to write a fan-fiction story with permission directly from the creator of the setting. I backed away from even that because it might be considered in the grey area of what Tapas considers "original". When dealing with a contest, you have to remember that you will be competing against others who are working entirely within the rules and will feel very salty if they lose to someone who they might consider cheating.
I guess that nothing is stopping you from going ahead and participating the way you want in the community project and using the hashtag, but I doubt you would be considered as a serious contestant if you continue a story which was started and published before the beginning of the contest.