Manually add a method to a DLL - Possible?

[Marton]

Is there a way to add a method to a DLL?
I want to use a DLL of an older version of a target, in a newer version. On startup I get an error:
Method 'IsSecondInstance' in type 'App.Activator.Activator' from assembly 'App.Collector, Version=1.0.4198.19407, Culture=neutral, PublicKeyToken=null' does not have an implementation.
So I want to add "IsSecondInstance" manually, is it possible? Any tools to recommend? Thanks!