How you adding CC field for first message? If you add as participant in first message, it should automatically add to replies as well.

Can you please give me screenshot with marking from where you set “CC” field?

Did you already added CC field there (I see in your previous message)? If yes, how you added that?

If no, do you want to give user an option to add cc there? Or you want to add automatically? If automatically, what is there user id which you want to add as a CC for all messages? So that i can write some code and give that to you which will automatically add that user to all messages for you.

Will it add automatically to the replies or you will add it manually?
If automatically add to all replies, i can give you code and you can add in your website.
If you need to add it manually (may be a select field with all users from where you can select one) then it will need custom code which may need fees.

Please contact using https://www.shamimsplugins.com/hire/

Plugin customization is not included in paid version. after that also i try to provide code if it needs small code snippet.
But to achieve your required functionality it not only needs template editing but backed code as well.

If you know php you can follow following steps
1. Edit view-message-content.php template file to show select field of users with search capability.
2. Add an admin ajax hook or backend function to handle selected user
3. Set that user as participant of that message using insert_participants method