You can add them No problem. Might be a little pecking order adjustment but it will not be a problem. I've also added younger chicks. With 5 week olds Id go as young as 2 week olds. The only problem I can see is a Bio-security issue. If they are coming from another farm a 30 day quarantine period may be recommended. If coming from a hatchery you can add them without quarantine.
Just an update. Younger chicks are doing great. The older ones seem to be keeping them warm and taking great care of them, no pecking order at all (I have baby cam in the coop so I can see what's going on all the time).