Example of the Powershell script which can modify IE LAN Proxy Exception List to keep current settings (for example pulled by GPO) and extend with custom exception list.Most important is to modify first line.

Script:

$ProxyExceptionList = ";*.google.com;*.microsoft.com"

$ProxyProperty = Get-ItemProperty "HKCU:\Software\Microsoft\Windows\CurrentVersion\Internet Settings"

If ($ProxyProperty.ProxyOverride) {
    $OldValue = $ProxyProperty.ProxyOverride
    $NewValue = $OldValue+$ProxyExceptionList
    $ProxyProperty | Set-ItemProperty -Name ProxyOverride -Value $NewValue
} else {
    Write-Warning "Proxy overrides not set!"
}