Can I add more chicks to hen with older chicks?

The hen should attack the new chicks. Raise them separately and combine the groups when they are added to your flock.

 

Rarely, some folks with super broody birds like silkies have done this, but overall it's a poor idea. At 2 weeks, momma knows who her babies are and has no instinct to mother any other littles. She's likely to see them as competition for resources and attack them.