You can create a new custom company property (single-line text) to store company email addresses. You can also customize your company record sidebar to show this new company email address property right at the top of your record.
However, you won't be able to use this email address in website forms because forms can only use contact properties. (There is a workaround for Professional and Enterprise portals using workflows: You would create the company email address property both on a contact-level and on a company-level. The form could then display the contact property, a workflow could copy the information into the company property.)
If you want to use the field in forms and don't have access to workflows, you'd have to create a custom contact property (and this information would not be available on the company record).
Also keep in mind that email marketing is limited to contacts. Even with the company email address stored in a new field, you wouldn't be select a list of companies as recipients.
Hope this helps!
Karsten Köhler HubSpot Freelancer | RevOps & CRM Consultant | Community Hall of Famer