Yes, unfortunately you have missed out. However, it is not inconceivable that there could possibly be some form of kickstarter like pre-order package for D:OS before the release of the game.
With the pledge management system on the Larian Vault, there is no way to add more money for add-ons and no way to upgrade the tier, due to the amount of work that would take compared to the number of people expected to take advantage of that feature. When the management system went live, though, there were a bunch of requests for that. After Dragon Commander is released, it is possible Larian may reconsider that decision. If they do decide to allow tier upgrades, then AFAIK it shouldn't be terribly hard to also add some kind of slacker backer tiers, as well.
Welcome to the forum.
