You can carb the keg warm or cold. Depends on what your process is. Remember that a keg is a just one large bottle and should be treated as such. If you normally condition your beers for an additional time period, then by all means do it with the keg. If you have the capability to hook the keg up to CO2 while this conditioning is happening, that is great. But, because the beer is warm, 15psi isn't going to cut it. There are plenty of charts on line, but for example.... to use the 'set it and forget it' method to carb at 65F, you'll need to set the regulator at roughly 25psi for 3 weeks. My pipeline is full enough that I can do just this. I keg my beers, purge and then carbonate them warm while they condition in my basement. This way when I blow a keg, I have a carbed one ready to go. Remember, it isn't a race to get from grain/extract to glass. Patience will not only make your beer better, but you a happy brewer.