They don't work as a plug in, but you can use them as a stand alone application as I remember. Open, say SilverEfex, go to file and open desired image. You can then work on it and save it back to then open in Xara should you wish. Not as convenient as a plugin but just the same results.
Attached image was produced in SilverEfex and opened in XaraDesigner etc.
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