You can add an individual ticker before 20 days, but you just can't have a signature before 20 days. You have to have 25 posts before you can post the ticker.
Go to Quick Links up top
then Sig Weight Tracker
then make your ticker and post it in a comment until the 20 days are up and you can have a signature if you're dying to use it!
Hope this helps! Here is mine to show that you don't have to be a member 20 days to just post it in a comment.