You cannot add a user to the SharePoint_Shell_Access role in all content databases once by PowerShell.
Here are examples:
1.Add a new user named User1 to the SharePoint_Shell_Access role in the farm configuration database only.
Add-SPShellAdmin -UserName CONTOSO\User1
2.Add a new user named User1 to the SharePoint_Shell_Access role of both the specified content database and the configuration database
Get-SPDatabase | ?{$_.Name -eq "WSS_Content"} | Add-SPShellAdmin -Username CONTOSO\User1
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