New silverlight/C# app - advice please

Hello,

I am re-developing an web application that has been written in Delphi/Intraweb.

We are moving to Silverlight .Net 4 and C#.

The web application does a simple job of collecting and validating data via a web page.  

The fields on the page, position, labels, the data type, and where the data is stored is all configurable.   We can even specify a number of tabs and define which field is on which tab.  All this is dynamic and the pages are created at run time.

 So to add a new field we just need to modify a few configuration variables and "hey presto" the new field appears on the web page nicely positioned as a text or combox box etc. the next time the user runs the app.  

This is not rocket science however when re-writing this application in C#/Sliverlight/.Net we have an extra requirement.

Now we want to add a tab on the page to handle some customised processing.  For example we might want to use a tab to display some charts or other input types when there is a need for some customised processing server side.

Basically we want to build a general purpose application that has a hook where we can attach some extra code without having to recompile the original app.

I am an old Delphi/Win 32 programmer.  If i was going to do this myself in that environment i would just write a dll with a standard interface.  My main application could load and call it if it was present.  

However I know very little about Silverlight/C#/.Net - so how would i do this in the new environment.

/Thanks