Here is a tutorial which shows how you can add more domains to TMail. Basic steps are as follows:
1. Add your new domain to cPanel (or any other panel) as Addon or Park / Alias Domain
2. Setup Default Address for that specific domain and forward all mails to same catchall id which you created while setting up TMail.
3. Add your new domain in TMail Admin Panel under Domains.
4. There is no step 4, that's it!