As of right now, the only way you can buy Steem (to transfer to steempower), is buy purchasing it with another cryptocurrency. You can buy either LTC, ETH, or BTC with your USD and then transfer the crypto you purchased to Steem. If you buy $50 worth of Bitcoin then you can use that $50 worth of Bitcoin to buy $50 worth of Steem. I hope that makes sense :)
RE: How To Buy Steem with USD (Newbie Guide 👼🏼 )