Interface injection
.net, c#, interface
Solution
I don't completely understand what you mean about injecting an interface, but you could use the Adapter pattern to achieve this. See also: http://dofactory.com/Patterns/PatternAdapter.aspx
Create your own interface, then create your own classes that implement the interface, which contain/wrap the 3rd party classes.
Problem
Is it possible to inject an interface into an existing 3rd party class that I can not alter? Like extension methods but for an interface (and its implementation for the class that it had been injected to). I like to optionally use one of two similar 3rd party libraries by giving classes that are similar in both libraries the same interfaces. So that I do not have to convert there classes into mine.