Put in a Defeat Boss objective. There should be an option to type in a new name for that particular boss.
As well as select the exact type of boss from a standard group, and (pet peeve when it's missing) type up a new mob description up to 300 chars long.
Invictus Est Level 50 Invul/Fire Tank
Malentis Level 50 Ice/Energy/Leviathan Dom (Freedom)
Black Jeremiah Level 50 Fire/Fire/Mu Dom
Sejanna Level 50 Dark/Dark/Elec Def (Virtue)
Arc #119664 - The MiniMech Cometh - Hess TF Mini-Sequel
yep, also make sure you are looking at the standard tab when looking for the boss. I have done that more than a few times 
Thank you for both quick replies. I tend to build/create modify my villain groups first and then attach them to mission attributes. This technique should handle the situation well that you have described.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
I didn't know this could be done. I was in a MA mission and the boss was a freak tank but he had a custom name. This is something I would love to be able to pull off: not necessarily tweaking the standard boss' powers but just to rename him. Please tell me how.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.