Yes you can! You actually got a Steam copy when you made the purchase! To claim it please follow these instructions:
1. Search your email for your order from SendOwl. It will contain a link where you can access your downloads and product keys (including Steam key).
2. If you can't find that email any more, you can get SendOwl to resend the email by visiting the following link: https://www.sendowl.com/order_recoveries/new?merchant_id=1228. Enter your email address and SendOwl will resend the product order emails.
IMPORTANT! You need to enter the same email that you used when you bought the game in order for the above to work! Also, please note that we cannot provide you with a new Steam key if you've changed email/can't recover it as we have no access to the system.