RE: Event-Portal and other functionalities
Hi,
First of all, they are two different types so they cannot be integrated with each other in its original form. Even their logins are different, the Customer Self-service Portal has a way to log in with an Azure AD account, while the Event Portal does not.

Please refer to this post for more details:
(+) Different Types of CRM Portals - Microsoft Dynamics CRM Community
However, if you just want to integrate, for example, some of the functions of the Customer Self-service Portal into the Event Portal, it is feasible.
You can customize the Event Portal directly through the Portal Management Application or Power Apps.
For example, integrating the case submissions to the Event Portal, it can be achieved by insert an entity form into the portal.
Define entity forms - Power Apps | Microsoft Docs
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya