Adding new quail to an existing flock can be done if done carefully. The new bird or birds should be at least 6 months old if you are adding to an adult group. If you have juvenile birds less than a year, you can add younger birds.
Make sure when adding the new birds you have plenty of room available for the new ones to hide and escape the original birds. So you might want to add branches, and obstacles to get around so that the new birds have a chance. There may be some fighting, so watch them carefully for a few days and be prepared to make new arrangements.
You can also put up a divider so that the new birds have their own space, the original birds can see them and let them stay this way for a few weeks. That way they get used to each other from the other side of a piece of wire. This is also a good idea so that there is some what of a quarantine period in case someone has an illness. Good luck!