It's tougher to just add one hen, could you persuade them into an extra one or two? Then pop them into the house at night and hopefully when they wake up in the morning, the newcomers will just be accepted. Longer term it might be worth getting a small run too. Means you can isolate new stock which is worth it just be sure they're not bringing in something horrid - and it also means you can pull out sick or injured hens from your own flock if they need it.
Enjoy your henkeeping!
H