When Adding a site to the QuickLaunch, we need to create a New-Object using Navigation.SPNavigationNode. Here’s an example on adding a site to the QuickLaunch.

function Add-SPSiteToQuickLaunch([string]$url, [string]$Site, [string]$SiteURL) {

  [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")

  $SPSite = New-Object Microsoft.SharePoint.SPSite($url)
  $OpenWeb = $SpSite.OpenWeb()

  $QuickLaunch = $OpenWeb.Navigation.QuickLaunch
  $Node = New-Object Microsoft.SharePoint.Navigation.SPNavigationNode $Site, $SiteURL, 1
  $QuickLaunch.AddAsLast($node)

  $SPSite.Dispose()
  $OpenWeb.Dispose()
}

Add-SPSiteToQuickLaunch -url http://wss -Site TheBlog -SiteURL http://wss/TheBlog