Hello Fady,
“TO” and ”FROM” fields in email can be filled only with Lead, Contact, Account or User.
So you cannot populate “TO” of “FROM” fields by attribute from custom entity by workflow.
We had similar problem, where we needed to send an email to custom entity called Demonstrator, and we solved it using custom plug-in which created temporary contact with email and name from Demonstrator custom entity and then adding this new contact in “TO” field in email. Also in the regarding field of email we inserted lookup to Demonstrator entity. After sending an email plug-in deletes newly created contact.
This solved our problem, but someone maybe knows some different workaround.
Vladimir