Modifying Existing .NET Assemblies

Question

Is there a way to modify existing .NET assemblies without resorting to 3rd party tools? I know that PostSharp makes this possible but I find it incredibly wasteful that the

Answer 1

It would help if you could provide a more specific use case, there are probably better ways to fix your problem than this.

In .NET 3.5 it's possible to add extension methods to existing framework classes, perhaps that is enough?