If she started pecking them, then it's probably a no-go at any time. She might be a bad broody and should be put in broody jail for a couple of days and never allowed to brood again. How long has she been sitting for? Though how long a broody has been sitting never affected my broodies when offering them chicks, perhaps it's the case here? That is, has she sat for 21 days yet? Also, did she have any of her own chicks previously?
Whenever I've snuck chicks under a broody, I've done it in broad daylight and watched them. If the broody is good with the chicks (sometimes she'll try feeding them), then she's good and they're all set. If the broody is bad with the chicks (pecking them, etc. I had one case that tried killing the added chick), then she's a no-go. If she's not a proven brooder, then she shouldn't be trusted with her own hatch. If she is a proven brooder, then she's just trying to kick out the "mice" (strange chicks).