Yes, as long as your other websites are owned by you and belong to the same company, and the products/services you sell on these websites are similar in nature to those sold on your primary site.
Be aware that only one trading name may be attached to your merchant account, and that this trading name is what any customers who make payments to this account will see on their credit card statements. Depending on the nature of your different websites, it may be prudent to set up separate merchant accounts to reduce the risk of chargebacks.