Sounds like you decided against it, but I agree, I wouldn't risk trying it, she isn't committed to being broody and the chicks are also possibly too old at this point to take to her anyhow. Sometimes it works out regardless, but it's less likely the less committed she is, and the older the chicks are.
In the future, if you do want to try slipping some chicks in, you really want the hen to have committed for around 3 weeks or so, and you want the youngest chicks you can get, and that should give you the best odds of success.