Inherit DLL Page With Additional CodeFile?

Question

I'm just about at my wits' end. I found this FANTASTIC simple file management tool called Webfiler here:

https://github.com/SeanMollet/webfiler

And it works great. HOWEVER, I have a need to add my own custom code to the ASPX.cs file. Well, when I load the DLL to reference it and bring in the sample ASPX file that came with the project that references the DLL, it doesn't appear to have an ASPX.cs file at all. I'm guessing that it's because the ASPX file is referencing the WebFiler DLL for its CodeBehind. Which is fine, but then if it's doing that, does that mean that there's no way for me to "inherit" that DLL WebFiler object and still add my own additional CodeBehind ASPX.cs file? When I try to forcibly add my own ASPX.cs file, and put a stop in the debugger, it never stops at my stop point, which I'm guessing means that the IDE is just ignoring my artifical .CS file entirely. Here is where you can see what I'm doing, the code that the IDE just won't stop at, it ignores this entirely, even though as you can see I'm trying to inherit from the WebFiler.Filer class:

The only solution that I see is just outright importing the project code itself and modifying it to do what I want it to do - which I really don't want to do, because I'm using this same WebFiler for two different subdomains, so I'd rather just make use of the DLL without having to have two completely separate copies on my site! Is this possible? Can a .cs file for a page "inherit" a DLL file and still allow for additional code on top of what the DLL provides?