Yes, you are able to add additional contacts to an Individual Customer's profile. This is to help properly track who is actually approving estimates and recommendations.
To add an additional contact simply search for the customer profile, open it up, then select the pencil icon next to 'Contact Info' to open up the Customer Edit modal. From there, select the 'Add Additional Contact' button to enter in the new contact's information.
Once the additional contact's information is saved, you will see them listed on the customer profile below the customer address section.
See also, Sharing and Approving with Multiple Contacts