You must create a friendly assembly, but you would need to recompile that third party assembly:
A friend assembly is an assembly that can access another assembly's
Friend (Visual Basic) or internal (C#) types and members. If you
identify an assembly as a friend assembly, you no longer have to mark
types and members as public in order for them to be accessed by other
assemblies.
You can use reflection though: How to access internal class using Reflection
Not recommendable anyway.
