I did that this year. I have a banty cochin who successfully hatched and raised 4 chicks last year.
I wanted certain chicks this year, 4, which I ordered. I put some eggs under her about the same time I ordered the chicks, which were due to arrived in a month. She went broody within a week, but there wasn't much hope for the eggs under her, which was ok with me. When the 4 chicks came in, during the daytime, I slipped a chick under her and removed an egg. I think at this time she had 5 eggs under her, which were all bad. I did the same with the other 3 chicks, one in, an egg out, and she took right to them. It was wonderful.
However, and food for thought here, shipped chicks, have more tendency to develop pasty butt, than chicks raised by a broody. My setup was not good for checking the chicks daily, especially since she would herd them away from me, and I lost one 2 days later to pasty butt. I then went to the local feed store and bought 4, what they said were day olds, and had I looked closer before I got them home, I would've realized they were closer to 7 or 10 days old.
I tried slipping them in, it worked ok for the first night, she was alright with the that next day, but wasn't mothering them, but kept them warm the next night. The following day, she was totally rejecting them. She wasn't outright mean, she kept her 2 chicks in a corner, the new 4 stayed in the nest box, and anytime they tried to come out for food and water, she would peck at them and chase them away.
My 3 twelve week old banties in another part of the coop, ended up raising those 4 chicks, but that's for another story.
I really think she would've taken in the 4 new chicks, had they been as small as the first 2 she took.
And remember, these first chicks came in about the time her eggs would've hatched. She also loves to be a mama hen, and I almost think I could slip a few day olds in with her even if she wasn't broody, but that's not one I'm going to try.
Just make sure if you add more chicks, it's within a day or two, and they are all the same size and age.