Sure. We found the following snippet on the NFA Upgrade guide:
If you unregister CA Network Flow Analysis from CA Performance Center 2.0.x or CA NetQoS Performance Center 6.1 and register it again with CA Performance Center 2.2.x or 2.3.x, the following rules apply:
....
User accounts: – User accounts are retained if the users have valid product privilege settings (User, Power User, or Administrator) for CA Network Flow Analysis. User accounts with no product privilege for CA Network Flow Analysis are deleted
So, on the NPC side we edited the users from the Admin > Users section, and assigned them a role (or Product Privilege) in NFA, like in the image.

After assigning the role we unregistered the NFA rom the NPC and registered it in CAPC. As those users had the NFA role, they were not deleted when we unregistered the NFA (the other users not related to the NA were not retained).