There is a possibility add an email address for custom domain in case the user owns it (business domain).

I am using a eternal email relay address provided by non-profit student organization. It has an email address which I can forward to any email provider I want. The service itself doesn't have any mailboxes.The incoming mail forwards just fine to Proton Mail. I would like to also send emails using this relay email address as "from" address. This is not possible currently as I don't own the domain.

I could set up this sending address in Gmail but now that I want to fully transfer to Proton and have paid for the subscription, this lack of feature is really troublesome.