Yes, of course. When you create resources, each has his/her own calendar. If Bob only works 6 hours per day - then change his calendar. However, if you want to say Bob can only produce 6 hours per day out of 8 hours - then don't change his calendar, set his max units at 75%. Then, when you assign Bob to the task, he will be assigned at 75% - the work for him will be calculated at 75% of duration. Or, you can enter his work and Project will calculate duration.