The may know each other already if they grew up together. Did they? This could lesson the impact. There will be disruption to the pecking order. The risk of biosecurity is somewhat lessened by them having been together so recently. How old are they?
There is a trick I have used in the situations that can lesion the impact.
Do not introduce them during the day, rather simply put the new hen on the roost when it is really dark. They all wake up together and it can be just like she was always part of the group.
It is important that she have room to get away if the others react badly or you be observing in case you need to intervene. Overall it can be done and I have done it with little fuss several times.
One thought, if it went really bad, could you return the hen if you weren't comfortable with how it was going?