Can I add 1 primary node in existing cluster?
The architecture of an availability group is that there is one primary node that owns all databases in the availability group.  A listener has an IP address, and it would be impossible to have that IP address online on multiple nodes at once.

The only way you can use another node (NodeC) as a primary while the current node (NodeA) is also a primary is to create an additional availability group.  You could set up this new availability group so that only NodeC and NodeB are server replicas, or, you could also include NodeA as a replica as well if desired.