How to change unknown senders to known when no phone number

I have 'Filter Unknown Senders' switched on in my iPhone settings to filter out spam text messages. However, some of the messages that come through are from legitimate senders that I want to see, it's just that the source is obviously from their online systems and don't have a phone number attached. I know how to add senders as a new contact or add them to an existing contact but when they have no phone number this isn't possible. Is there a way of adding known message senders so they're not filtered out when they don't have an actual phone number?