$Names = @{
    Willem = 'Van Oranje'
    Jan = 'van de Vecht'
    Piet = 'Paulusma'
}

ForEach ($Row in $Names.GetEnumerator()) {
    $LastnameIndex = $Row.Value.LastIndexOf(' ')

    If ($LastnameIndex -lt 1) {
        $LastName = $Row.Value

    }
    $LastName = $Row.Value.Substring($LastnameIndex+1)

    $Username = $Row.Name.Substring(0,1) + $LastName
    Write-Output $Username
}