She's broody but she is not trying to hatch any longer now that she has the chick. She is now brooding the chick.
The ideal way to graft multiple chicks to a broody hen is all at the same time with the same number of chicks as eggs she had.
You can try to give her a couple of chicks and see if she'll accept them. But there is a fairly good chance she
Has she been broody in the past?
How old is the chick?
How old are the chicks you want to introduce?
If you do it, as Sour mentioned, giving her more of the same color chicks might help. I would slip them under her at night and wouldn't try to give her more than 2 more. Be out there well before sun-up to monitor and intervene if she rejects the new chicks