I have satellite assemblies pre-built and I want to add them into the silverlight xap file after the file has been built. I know if I do this manually all I need to do is add the files into the xap (it's a zip file) and update appmanifest.xaml.
Are there any tools out there (or msbuild task) that I can call in the post build process?
you can dynamically load dll's at runtime. Have a look here: Dynamic assembly loading
Have you considered using MEF? Here's a linke where they show how to download XAP's at runtime. Silverlight TV 11: Dynamically Loading XAPs with MEF.
I hope this helps you.
BR,
TJ