I have added feedstore chicks to broody hens that hatched their own chicks many times. That is exactly the way I add new breeds to my flock.
I have learned a key trick to ensure a 100% broody acceptance rate. To answer your question, yes give the feed store chicks to your broody at night, but wait until After she has finished hatching her own chicks. The reason is the feedstore chicks will be a few days older, & will need to eat and drink the next morn even if the broody's own chicks are still hatching. You don't want to cause your broody to abandon her own eggs to attend to the needs of the older feed store chicks. On the day you bring the feed store chicks home, place them in a temporary brooder complete with food, water and heat.
To make sure your broody hen doesn't reject the feedstore chicks, it is imperative to add the feedstore chicks to her own hatchees Before she brings her own chicks out to eat and drink for the very first time. Otherwise, she may recognize the feed store chicks as not her own and reject them. After her chicks have finished hatching, add the feed store chicks in at night to sleep with her own chicks. The chicks will all get cozy and comfy together under mama hen, & the next morn she won't know the difference between her own and the ones you gave her to adopt.
I have had 100% success with this method, even with broody hens that previously rejected feed store chicks. The key to 100% acceptance is to add the adoptee chicks during that short window of opportunity.
Also, there is no need to quarantine feed store chicks that come from a reputable hatchery, and as long as the feed store takes basic biosecurity precautions. Most all feed stores do so. If they don't, it's advisable to not buy chicks from that store.
Have you candled the eggs under your broody to see if they are developing? If any are not, they should be removed and discarded lest they rot and explode under the hen.