No, you can't. Any amount of coins sent to @null is effectively burned, meaning it is destroyed and removed from the blockchain forever. So, there is no way to recover it.
Let's get things clear. The @null account is not a real person or a user on the Steem Blockchain. Instead, it's a special blockchain level account. No one owns or controls it, thus there is no one to contact about all the SBDs sent to it, whether accidental or not.
What happened to your SBD? As the Steemit.com FAQ explains, "Once a user transfers SBD to @null, the Steem blockchain removes them from the currency supply." Thus, your SBD is gone. No one has it, and there is nothing to recover anymore.
The thing is, the "Promoted Posts" tab in Steemit.com isn't really an effective way to get exposure for your posts. No one visits that tab. This is probably why you didn't get much exposure from using it. Unfortunately, Steemit Inc. is clear on their disclaimer that the @null account does not belong them. And that sending money to it doesn't constitute a contract between you and anyone else. So, there's no one who could be accountable for the loss of your SBD.
On a more productive note. If you have a question, Musing.io is really the best place to ask it. There are more than enough users here who pay attention to almost all the questions being asked. So, as long as you're asking a legitimate question, someone is bound to help you with an answer here.
Hope this helps.
Whatever you send to anyone on steemit, you can't get it back unless the person decides to send it back to you. That's why you have to be sure of a transaction before making it. You can send a memo to the account you transferred the sbd to and plead with him/her to send back your sbd, that's the best you can do