If you follow the article on Microsoft on topic – New-StoragePool you will find out that it just takes all available disks that can be pooled into a variable.
If you want to add just some of available drives into a pool you should create arraylist of disks which you can populate by using just some drives identified by its UniqueId.
Get-PhysicalDisk -CanPool $true | ft FriendlyName, Size, UniqueId
[System.Collections.ArrayList]$disks = @(Get-PhysicalDisk -UniqueId 60022480015BE59BBBFBD5FBAF9A69DE)
$adddisk = Get-PhysicalDisk -UniqueId 60022480C6BE12F96745D63C63131D0D
$disks | ft uniqueid