In order to allow customers to use a discount code on top of an Ultimate Special Offer discount, you'll need to enable the settings within the Ultimate Special Offers settings page. There are two ways to use discount codes on top of an offer: before the offer or after.
Before: If discount code requires a minimum purchase to be valid, the larger cart total before special offers is used
After: If you use Spend Amount or Free Gift, offers will apply even though total will go below the required amount after using a discount code.
Example: Spend $50 and get a Free Gift. Customers spend $50, they get the gift, and enter a code with 10% discount -- total will be $45, and the gift will still be free.