I don't recommend this as it's against Kentico's advice but you could:
- Create a FormControl which validates for your example.
- Go to the database, find CMS.Class Where ClassName = 'cms.emailtemplate' and up change ClassShowAsSystemTable to false
- Go to the CMS.Email Module and update the Email Template class to use the new form control for the from address.
Ofc fully test after!