$ConfigurationData = @{
    AllNodes = @(
        @{
            NodeName="*"
            PSDscAllowPlainTextPassword=$True
            PsDscAllowDomainUser=$True
        }
    )
}

$ConfigurationData = @{
    AllNodes = @(
        @{
            NodeName="*"
            PSDscAllowPlainTextPassword=$True
            PsDscAllowDomainUser=$True
            NewItem = "SomeNewValue"
            AnotherNewItem = "Hello"
        }
    )
}

$ConfigurationData.AllNodes += @{NewItem = "SomeNewValue"}
$ConfigurationData.AllNodes += @{AnotherNewItem  = "Hello"}

$ConfigurationData.AllNodes

Name                           Value
----                           -----
NodeName                       *
PSDscAllowPlainTextPassword    True
PsDscAllowDomainUser           True
NewItem                        SomeNewValue
AnotherNewItem                 Hello

$ConfigurationData | ConvertTo-Json
{
    "AllNodes":  [
                     {
                         "NodeName":  "*",
                         "PSDscAllowPlainTextPassword":  true,
                         "PsDscAllowDomainUser":  true
                     },
                     {
                         "NewItem":  "SomeNewValue"
                     },
                     {
                         "AnotherNewItem":  "Hello"
                     }
                 ]
}

$ConfigurationData = @{
    AllNodes = @(
        @{
            NodeName="*"
            PSDscAllowPlainTextPassword=$True
            PsDscAllowDomainUser=$True
        }
    )
}

#$ConfigurationData.AllNodes += @{NewItem = "SomeNewValue"}
#$ConfigurationData.AllNodes += @{AnotherNewItem  = "Hello"}

foreach($Node in $ConfigurationData.AllNodes.GetEnumerator() | Where-Object{$_.NodeName -eq "*"})
{
            $node.add("NewItem", "SomeNewValue")
            $node.add("AnotherNewItem", "Hello")
}

$ConfigurationData | ConvertTo-Json
{
    "AllNodes":  [
                     {
                         "NodeName":  "*",
                         "PSDscAllowPlainTextPassword":  true,
                         "NewItem":  "SomeNewValue",
                         "AnotherNewItem":  "Hello",
                         "PsDscAllowDomainUser":  true
                     }
                 ]
}

$ConfigurationData.AllNodes.GetEnumerator() += @{"NewItem" = "SomeNewValue"}

$ConfigurationData = @{
    AllNodes = @(
        @{
            NodeName="*"
            PSDscAllowPlainTextPassword=$True
            PsDscAllowDomainUser=$True
        }
    )
}

$ConfigurationData.AllNodes.GetEnumerator().Add("NewItem","SomeNewValue")

$ConfigurationData.AllNodes.GetEnumerator().Add("AnotherNewItem","Hello")

$ConfigurationData | ConvertTo-Json




{
    "AllNodes":  [
                     {
                         "NodeName":  "*",
                         "PSDscAllowPlainTextPassword":  true,
                         "NewItem":  "SomeNewValue",
                         "AnotherNewItem":  "Hello",
                         "PsDscAllowDomainUser":  true
                     }
                 ]
}