Are you referring to the Azure portal or the Microsoft 365 Defender portal? This is possible but it will take a bit of effort.
CS alerts can be forwarded using Syslog-CEF. They have a forwarder and Microsoft has a collection agent. So you can forward CS alerts to a Log Analytics Workspace.
The following article describes the Microsoft forwarder. Sentinel is not required. The same instructions work with a standard workspace. You point the CS forwarder to this IP address.
The M365D portal does not have a custom dashboard option. You can query the M365D advanced hunting logs using an API. PowerBI is another option. You can also forward the M365D alerts to a log analytics workspace in Azure. Sentinel has a built-in M365D connector. The alerts can be sent to Sentinel for free.
Once you have M365D and CS alerts in the same Log Analytics workspace, you can create custom workbooks and dashboards in the Azure portal. You could also call that workspace with PowerBI.