No, Expression Blend does not support third party plug-ins but I have a good feeling it will in the future. After all, their competitors all allow plugins so they will have to also to complete for market share.
We use Visual Studio with all of our blend projects so yes, it does work. Blend makes it easy to switch between VS and back again to that you can add increased functionality to your projects using c#. There is a tutorial on Lynda.com that gives clear instruction on using VS with Blend.
Hit this link and scroll to the bottom of the page.
http://movielibrary.lynda.com/html/modPage.asp?ID=384
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Deploying with Visual Studio