You can actually skip through ranks if you have potential activity once you've made the required posts. For instance, this account had the potential to be a Hero rank even when I was a Full Member once I'd made the 480 posts to get there. You can only do this by posting in 35 activity periods though so it's actually possible to go from being a Junior Member straight to Hero but you would still have had to make at least one post in each activity period over the time it would have taken you to naturally get there (and then the 480 posts) so this is obviously not possible for your account.
The only other alternative is to buy a forum account (not recommended) but this is why I've previously suggested users be able to donate to the forum instead to get essentially a higher ranked account with larger signature etc.
Edit: More on potential activity here:
https://bitcointalk.org/index.php?topic=1655539.0Though searching 'potential activity' will bring up plenty of other threads explaining it.