Don't know how big your chimney is, but a full one should do fine for 1 hour. Depends a lot on air flow of course, but that's a decent load.
Adding coals midway is an interesting topic. The snake and minion methods have long been used with zero issues, which is where you only light some of your coals, and the rest catch light mid cook. So that's an argument that you have nothing to worry about, whether charcoal or briquette. However, in that case the coals are being lit one by one over time, not a sudden hit of lots getting lit at once when you drop a load unlit onto a bed of fully lit coals. There is potential that adding a dose this way will give you a bit of a hit of smoke.
What's also worth thinking about is what are you trying to achieve with the extra coal. Adding unlit coals can actually temporarily decrease your temperature, whereas adding lit ones is an instant boost.
So for instance if you are midway through a long low cook with a heat deflector in place, and just want to prolong your cook, banging in a load of unlit ones won't give you any issues. Your in place coals aren't roaring and your heat deflector will help a bit with diverting the smoke, and the short time temp drop isn't a worry.
But if you have a roaring bed of coals, adding a load of unlit ones will mean they all catch at the same time resulting in a fair bit of smoke, and it also might dim your fire for about 5 mins too, which might not be what you want.
Sorry that's a bit of a meander of a response!
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