Likely you are asking for trouble IMHO, even appearling healthy additions to a flock can bring a new strain of cocci, worms, mites or worse in the form of numerous other diseases. The second issue is new birds may not just fit right in and the reestablishment of a pecking order can cause injuries, reduce egg laying and gennerally up set things.
Far better to quarantine new additions for a month then introduce them slowly so they get a chance to see each other through an adjoining fence for a while before allowing them to mix.
You don't need to but if you rush things the risks are much higher. With hearing so many BYC stories of people bring sick chickens into a flock and ending up with major outbreaks it is likely a good idea to do a little reading about CRD, Coryza and some of the other problems before jumping into the deep end.
You may get away with it once but keep rolling the dice and sooner or later you will roll snake eyes.
