I'm going to split the difference here. If you are away from a computer and want to add just one or two channels to an existing Favorites List, then Menu>Manage Favorites and scrolling to the applicable "stuff" works fine. If you have add Systems/Departments or numerous channels, its more efficient to do it on a computer.
The critical thing to remember is if you have the Favorites List on your computer, be sure you use Sentinel after you make the change via menu on scanner and do a "Read from Scanner" so that the added channels are included with your computer files. Otherwise, they could get lost in a shuffle.
Once you get it down good its very fast, I have added whole systems that I have found on the fly, I usually only put the name of the system and then when I'm near a computer add the talkgroups, the alpha numerics are the time consuming part if your in the field adding a system, everything else is pretty quick.