Add-Ons

This question might be one of those which can't be easily answered but I feel it's worth asking to see what replies I get.

I want to build my application and I want to be able to allow for "add-ons" to be installed, My project will be a MDI application with many different windows being opened in it.

I want some users to have optional other windows (winforms) which can be accessed. This means I need to someone work out what "add-ons" there are and adjust the menu and also add extra winforms?

I assume, extra DLL's can contain my windorms? if not how would I add these?

How can I get my menu to read what add-on's there are at the time my main window loads?