The problem with using charcoal and a minion in an offset, especially a small one, is that it does not hold a lot of charcoal to begin with. Also, unless you have a weird burn pattern (such as I do in my UDS) you are better with the charcoal going in the middle and burning outward. Burning from one corner never lets the right amount of charcoal burn at any given time.
There is no reason other than a potential for off aromas, for not adding cold charcoal to a fire. I prefer to add heated charcoal if need be, as I feel it keeps the burn more consistent. Of course, I use a UDS and a kettle and not an offset, and that is a difference as well.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."