When Adding a site to the TopNavigationBar, we need to create a New-Object using Navigation.SPNavigationNode. Here’s an example on adding a site to the TopNavigationBar. This is basically the same as when adding the Site to the QuickLaunch.

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

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

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

  $TopNavBar = $OpenWeb.Navigation.TopNavigationBar

  $Node = New-Object Microsoft.SharePoint.Navigation.SPNavigationNode $Site, $SiteUrl, 1
  $TopNavBar.AddAsLast($Node)

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

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