カスタムオブジェクト

New-Objectコマンドレットを使う。

 PS> $users = @()
 PS> $u = New-Object psobject -Property @{ Name = "taro"; Age = 19 }
 PS> $users += $u
 PS> $u = New-Object psobject -Property @{ Name = "jiro"; Age = 15 }
 PS> $users += $u
 PS> $users | ConvertTo-Json
 [
     {
         "Age":  19,
         "Name":  "taro"
     },
     {
         "Age":  15,
         "Name":  "jiro"
     }
 ]

参考

http://stackoverflow.com/questions/15388367/create-custom-psobject-powershell-2-0