In Impulse, on the registrations tab, you can add a registration manually.
You then enter the email address used to purchase the game, and the serial number that is emailed to that address when the preorder is processed. It does not have to be the same email address as your primary Impulse account.
The game will now show up in Impulse.
Note that it is very important you keep track of the email address that purchased the game (if it's different from your Stardock.net account one). This is the address/account that the game will remain registered to.
If it was purchased as a gift, and not through the actual gift method (which would associate the product properly to the recipient), you can contact sales@stardock.com to get it sorted out.