There is no function to BCC someone into a ticket, by design. Consider what happens if a person who is BCC'd onto a ticket replies to a ticket message. If Deskpro added their reply to the ticket then all other users/agents could see the message, which defeats the purpose of a BCC.
If you want to create a separate email thread (outside of Deskpro), you can forward an individual message as an email using the gear icon on each message. Another option is to create a linked ticket and add the person you wish to BCC.