Set a custom object's value using reflection

Hi

i have an application that uses a dll and executes the methods inside the dll

inside the dll i have 

public class DllObject
{
   UnknownType custObj;
   public void UseCustObj()
   {
      Send(custObj);
   }
}

public class UnknownType
{
    public string name;
    public string address;
}

Now using reflection i need to set the value of custObj. UnknownType is not known to the executing assembly thus i cant declare an instance of UnknownType and set it to custObj. Any suggestions?