.NET expression engine or plugin framework

I need a .NET expert (ASP.NET) with knowledge of workflows / business rules processing. I have setup a very simple type of workflow engine (with sometimes long-running workflows), which connects tasks/inputforms. However sometimes routes needed to be conditional, e.g. a condition needs to be met before the next task is presented. Since the workflows integrate with a ASP.NET web application I need a flexible expression engine, so I can add business rules at any time without the need to recompile the application. As it should be able to integrate with existing fields from the application & database it would probably be ideal if some type of plugin framework can be used, so the expression engine is actually C# or VB.NET code, but alternatively if that's not possible I can imagine using something like Jint or other expression engine. Can you help me with integrating an expression engine and/or plugin framework for this case? Please mention your expertise and experience.