We have a custom object "Coupon" in Sales Cloud. This object has Contact lookup field and 'coupon code' text field. When Coupon record is assigned to a contact, s/he will become a campaign member (depends on the coupon code, contact will be added to different campaigns).
I am setting up the journey builder in Marketing cloud to send out emails to contacts with coupon code.
In order to get Coupon Code field value, I use Coupon custom object as a journey entry source. 
However, I also would like to update the campaign member status after email is sent. If I want to do so, I should use Salesforce campaign as a data entry source, and I cannot have coupon code field value as a merge field on email.

Could someone give me suggestions, please? Thank you.