Remember that whatever control you pass back from your add-in must inherit from System.Windows.Forms.Control… something none of the standard WPF controls do.
Instead to make WPF work in the console it's best to have the control your add-in return either inherit from or contain an ElementHost control which in turn contains all of your WPF goodies.
For note though… even though you can do this (so long as you've deployed the 3.0 or 3.5 Framework to your WHS)... running WPF inside of the admin console isn't a tested or supported scenario.
Maybe instead of having your add-in target the admin console with WPF… consider breaking it into three pieces. One a simple WinForms UI that is used for limited configuration/administration and then your larger WPF app, ideally running on the client machine (where any heavy UI intensive lifting should occur)… both of which that talk to some underlying data provider/service of your own that runs on the server.
