Eggs or day old chicks?

It's always a risk putting chicks with a hen that has been sititing on eggs (even "pretend" ones) as the hen needs to be tuned to "chick" mode in order to accept the chicks and this sometimes leads to the hens killing the chicks as they see them as interlopers and a risk to their eggs. You'd also have to wait the 21 days as the hen knows the normal hatching cycle!
I have fostered chicks under a broody many times, but the broody has always had a chick or two of her own beforehand and is therefore in "chick" mode. It would be better to put fertile eggs under her and let nature take it's course. If only one or two subsequently hatch, then you may want to consider fostering some same age chicks under her. She'll accept them more readily if she already has chicks of her own, but always put them under at night, hiding the chick under your hand as you place it under the roosting bird's wing. You'll also need to be there first thing as she wakes (very early at this time of year) in case she rejects the chick and tries to kill it. If rejection occurs, don't give up and replace the chick the following evening and so on. She will eventually take to the chick, but until this happens you need to be very vigilant at first light.

I have succesfully put abandoned chicks from an inexperienced hen under a broody hen successfully - however she is a seasoned hatcher and loves chicks (silkie cross). With the first chick she tried to peck it but I gave her beak a tiny tap and after that I put the other 7 next to her whilst removing her eggs (the eggs had been laid that day - she just kept insisting on sitting and being broody for weeks on end) and within a couple of minutes she was rosey away with them all under her